During the opening credits, the movie we are viewing is said to have been filmed in CINEMASCOPE (like the old-time musicals) and then the screen expands from box format to wide. We find ourselves on the 105/110 interchange in Los Angeles. People listen to various forms of music in their cars. Then one woman gets out of her vehicle and starts singing ANOTHER DAY OF SUN to camera (à la a musical), a song about how great it is being in sunny California. Someone else exits his car and joins her. Pretty soon, everyone has left their car and is singing the song while dancing in the street or on the cars that are parked via the traffic jam, everything synchronized for the full-length musical number.
The song ends and everyone gets back in their cars. We are told it is WINTER. Traffic finally begins to move. Inside her Toyota Prius, Mia (Emma Stone) appears to be talking to someone on the phone. In his classic car, Sebastian (Ryan Gosling) honks at her to move. We realize she isn't talking to anyone but is practicing for an upcoming audition. Sebastian goes around her, glaring, while she flips him off. She then murmurs that she should probably move as she's obstructing the traffic flow.
Mia works at a coffee shop on the Warner Bros lot, serving the employees of the studio. When a movie star comes in for coffee, the manager tells her it's on the house so she puts her money in the tip jar instead. Mia watches in awe as the movie star gets on a golf cart, being escorted to set. Then Mia's phone goes off, reminding her that she has an audition to get to. Mia rushes off so she won't be late and collides with a customer, spilling coffee all over her shirt. When Mia gets to the audition, she is wearing a jacket over her blouse. She does her reading, giving a really honest performance where she breaks into tears. But it goes unnoticed as someone appears in the window with a message. The casting directors interrupt Mia's audition while they talk to an assistant who tells them someone has called for them, which they say to relay that they'll return the call shortly. Mia is thanked even though they have neglected to watch her performance. She leaves the casting office and into the waiting area where everyone looks like her. Then she gets into the elevator where two other girls look like her and are wearing similar clothes, all presumably there for the same audition.
Mia returns home to the house she lives with three other aspiring actresses. They ask how her audition went but she doesn't want to discuss it. They are all going out to a Hollywood party at the home of someone in the film industry and suggest she goes along. She is reluctant but through a song called SOMEONE IN THE CROWD, Mia's roommates point out that someone might discover her if she attends and she can ascend from being just someone in the crowd. We see the party in a classic movie musical montage with close-ups of champagne being poured and creepy older men hitting on Mia. Time moves in slow-motion and the guests ballroom dance around her. The night has proven uneventful and Mia returns to her car only to realize she has parked in a tow away zone and her car is gone. She is forced to walk in heels down the canyon into the city. She walks around Hollywood and is distracted when she hears a beautiful jazz song being played on piano. It entices her to step into a restaurant/night club called Liptons, where she sees Sebastian playing the song.
We flashback to earlier in the day, when Sebastian is laying on his horn behind Mia in traffic. After he gets around her, Sebastian drives his car to a coffee stand. Across the street, he watches a club open for the day, that advertises Tunes & Tapas. He goes home to his run-down apartment, and his sister is there. He chastises her for putting her feet on a stool which he took from the street after a jazz club closed down, citing that one of his favorite jazz musicians had sat on it. In their conversation, it's evident that he loves jazz music and he tells her how his old club was closed down and turned into the Samba/Tapas place. His sister points out that all his bills are overdue and that he needs to start making money.
Later, Sebastian arrives at Lipton's for a piano gig. He talks to the club's manager (J.K. Simmons in a cameo role) about whether he can do some of his choices for songs. The manager refuses. He asks if he can do one as requested and then one of his own and is told no. He asks if he can do two as requested and then one of his own. The manager tells him all the songs should be as requested. So he begins to play Christmas songs, begrudgingly, as the patrons ignore him. After several holiday tunes, he begins to play his own piece. As fantasy, the lights in the club dim and he plays the piano in a spotlight. The piece (MIA & SEBASTIAN'S THEME) is beautiful. This is when Mia has stepped into the club. When Sebastian finishes, the fantasy fades and the lights come back up. The manager pulls him aside and tells him he's fired. Sebastian tries to convince him to give him another chance but is denied. Mia watches this whole exchange. As he passes her, she tries to tell him how moved she was by his music but he gives her a dirty look and pushes past her. This takes her aback and she is left speechless.
We're told it's now SPRING. We see a montage of Mia auditioning for different roles: Mia auditions again first for the role of a doctor, where she has to recite jargon that she doesn't understand; then the role of a police officer, reciting jargon she doesn't understand; and then she reads the role of a white woman trying to educate inner-city high school students, with the casting assistant reading the lines (''Why you trippin'?'') opposite her in monotone.
Mia ends at up at a pool party while an '80s cover band plays ''Take On Me'' by A-Ha. Mia is introduced to a young man who is a writer. He immediately goes into a spiel about how his career is blowing up and he's on everyone's radar; she excuses herself and ventures over to the band, only to see Sebastian is the keyboardist, wearing a hot red track suit akin to '80s fashion. The singer asks if anyone has any requests. She shouts out ''I Ran.'' Sebastian gives her a disapproving look, as the song only requires him to play the keytar. The band plays ''I Ran'' by A Flock of Seagulls' with the lyrics ''I never thought I'd meet a girl like you'' and ''I ran, I ran so far away.'' After their set, Sebastian finds Mia and tells her she can't request ''I Ran'' because he's a serious musician. She playfully mocks his outfit and says she needs his wardrobe for an audition she has that week, for the role of a Serious Fireman. He asks what she does in life that puts her so high above him to critique, and she tells him she works at a coffee shop on the Warner Bros. lot. He chastises her for being just a barista, yet talking down to him, but is then called back to the band for another set.
After the event, the writer tries chatting Mia up again. She sees Sebastian leaving and calls out to him to get her keys from the valet, as a way to escape the conversation. He asks what car she drives. When she tells him it's a Toyota Prius, he notes that all the cars are Priuses. She tells him the one with the green ribbon. We cut to them walking down the canyon among parked cars. She mentions it's strange that they keep running into each other, and Sebastian agrees, wondering if it means something. Mia says she doubts it. Her remote car key fob isn't beeping. He tells her to put it under her chin to connect with the antenna, joking that it will give her cancer but she'll find her car faster so it's a fair tradeoff. The two playfully bicker and then find themselves at the top of the canyon with a view of Los Angeles below, the sunset looking pink. They both joke that the view is nothing to look at. Sebastian begins singing A LOVELY NIGHT, a playful musical number where he says it's a waste of a lovely night on two people who aren't interested in each other. Mia changes from heels into dance shoes and midway through she joins him in the duet. They move from singing to dancing together. When the song ends, she gets a call from Greg, who turns out to be her boyfriend. She then puts the fob under her chin and hears a chirp. She gets to her car and asks if he wants a ride. He says, no, he's parked just up the hill. She drives away downhill. He watches her go and then also heads downhill, walking a long distance back to his car, which it turns out was parked right outside the house where the party was held.
Mia is back at work at the coffee shop. A snooty woman asks her if a pastry is gluten-free. Mia tells her no. The woman demands a refund. She tells her she has to get her manager to approve that. When she approaches the manager, she is told that she has to work Friday, despite having an audition. She goes back to the counter and Sebastian is there; the only info he had about her was where she worked. She asked how he got onto the lot and he said he hightailed it past security and might need to hide in the restroom for a little bit. He asks when she's getting off work and she tells him in ten minutes. The lady who wants the refund demands service so Mia tries to take care of it at the register before realizing she still needs approval.
Sebastian and Mia leave the cafe and walk along the lot; they pass many different sets filming varied scenes. They are stopped and directed around a live shoot, which prompts Sebastian to say they film movies on his street all the time. He asks Mia why she wanted to be an actress. She says her aunt was an actress and when she'd visit Mia in Boulder City, Nevada, they'd go across the street to the library and check out old movies. She used to write plays and put on productions and always wanted to act. As they continue walking past the stages on the Warner Bros. lot, Mia then tells him about her last audition for a show that is ''Dangerous Minds'' meets ''The O.C.'' He suggests that she write her own material so she doesn't have to suffer at auditions, citing that Louis Armstrong did this. She confesses that she doesn't like jazz music. This inspires him to take her to a jazz club called The Lighthouse. As the band plays, Sebastian tries to explain to Mia his love for jazz, and why he thinks jazz is great. He also tells her that jazz is dying, and his dream is to open his own jazz club where they play whatever they want, whenever they want. Mia returns a missed phone call in a quiet corner of the club, and discovers she has a callback. Sebastian asks what it's for and she said it's the ''Dangerous Minds/O.C.'' show but it's not as bad as she made it out to be now that she has a callback. It's more like ''Rebel Without a Cause.'' He quotes the movie, but she doesn't know what he's referencing, and he realizes she hasn't actually seen the film. He tells her it's playing at the Rialto and they should see it together later that week. She agrees and they say goodbye.
Sebastian is then seen walking along a pier at sunset. He sings CITY OF STARS, as he finds a man's hat on the pier, returns it to the owner, and spontaneously dances with the man's wife.
Mia goes to the callback for the show. One casting director texts on her phone while the other one just looks at her headshot instead of her. After she reads one line, he tells her "That's enough". She suggests doing it a different way but he won't let her. She is discouraged as she leaves and drives back home, but on her way she passes the Rialto, and begins to smile. Back at home, Mia is getting ready to go to the Rialto when her boyfriend, Greg (Finn Wittrock) arrives and reminds her they're going to dinner with his brother who just got into town. She doesn't have Sebastian's contact info so she isn't able to postpone their movie date.
Sebastian arrives at the theater, and waits outside for Mia. Meanwhile, Mia is at dinner while Greg has pompous conversation with his brother and his brother's date about all the exotic places they've traveled to. Mia looks up at the speaker, which begins playing MIA & SEBASTIAN'S THEME. She apologizes to Greg and leaves the restaurant, running toward the movie theater in heels. By this point, Sebastian has gone inside and the movie has already begun. Mia can't find Sebastian in the darkened theater so she stands up on the stage to scan the crowd, and Sebastian stands up once he notices her there. They watch the movie together, with the scene at the Griffith Observatory on the screen. Their hands slowly drift closer and closer until they touch. They hold hands. He leans in to kiss her and then the film breaks. The lights turn on and the moment is interrupted. Mia has an idea for a fun alternative.
The two end up at the Griffith Observatory, driving into the scene with a shot recreated from the film we saw moments before. They find an unlocked door around the back, and venture inside. They begin to dance around the halls and finally end up in the planetarium. Suddenly they float up to the dome ceiling, which is lit up with stars and find themselves in the stars. They dance in outer space, until they come back down to earth and finally have their first kiss.
It's now SUMMER. Mia and Sebastian are dating. She is writing a play, as Sebastian suggested. Her roommates ask for parts but she tells them it's a one-woman show. Sebastian arrives at her home but instead of coming to the door, he lays on his horn until she goes outside, annoying her roommates. What follows is a montage of the two of them on dates throughout the city, ending with the two of them at The Lighthouse, where Sebastian plays the piano and Mia dances with the other patrons. They sit down at a table together and are interrupted by a friend of Sebastian's, Keith (John Legend) who he had previously been in a jazz band with. Keith says his new band is looking for a piano player, and it pays, but Sebastian declines.
Back at Sebastian's apartment, Mia finishes performing her one-woman show for him, and he tells her it's incredible. She then shows him a logo for his club she made. She wants him to call it SEBS and he compliments that she used a music note for the apostrophe but he is set on the name Chicken on a Stick as an homage to jazz musician Charlie Parker, who got his nickname Bird because he loved eating chicken.
Later, Mia is on the phone with her mother. She is being asked about Sebastian and defensively tells her mom that he's going to open a jazz club but admits he doesn't have the money yet. She is insistent he is saving up though, even though he doesn't have a steady gig. Overhearing this, Sebastian is lost in his thoughts.
Sebastian decides to try out the gig with Keith and his band. Keith says they have a record deal, and a tour set up, and they can pay him $1000 a week, plus a cut of tickets and merchandising. Sebastian is momentarily speechless at this. They rehearse a song that begins as the type of jazz Sebastian is used to, then Keith adds in electronic, modern noises and Sebastian is thrown off, but he tries to continue playing. Keith pulls Sebastian aside afterwards and notes that Sebastian has always been difficult because he's a traditionalist but young people don't like that kind of jazz music it has to be modernized to appeal to them and Sebastian is foolish to cling to the antiquated past instead of focusing on the future of jazz.
We see Sebastian at home playing piano. Mia comes home, and Sebastian begins singing CITY OF STARS as he plays. Mia joins him on the piano bench and joins in, making the song a duet. As the piano music continues, we see a montage of changes in each of their lives. Mia quits her day job as a barista, and arranges a theater rental for her one-woman show. Sebastian signs with Keith's band, and proceeds to tour and do press events with them. We see that Mia is moving in with Sebastian, and selling some of her stuff, and working on her one-woman show, while Sebastian is out performing. They seem to be growing apart, as Mia is shown staying up late waiting for him, then Sebastian coming in after she's fallen asleep, and then Mia alone in the kitchen the next morning. The montage ends with a flashback of the two of them finishing their CITY OF STARS duet on the piano bench together.
Mia attends a concert for Keith and Sebastian's band, The Messengers. What begins as a jazz song turns into a dance number with samples of jazz elements (START A FIRE). The massive crowd of young people love it as it sounds very modern and radio-friendly. But Mia is surprised that Sebastian doesn't mind selling out when he is so particular in the style of music he loves. She searches his face for disapproval but he kind of shrugs the experience off as a non-issue. As the song and the crowd grows more frenzied, Mia is pushed to the back of the crowd, and fades out of Sebastian's view by the end of the song.
It's now FALL. Mia drives by the Rialto, where they saw ''Rebel Without a Cause'' and notices it's been closed. She emails a handful of people about her show, then on her way home she leaves a message for Sebastian, saying she misses him and that they haven't talked in a while as he's been on the road touring. However, when she gets home Sebastian is there to surprise her, having come back into town for just one night to see her. Over dinner, Mia notes that she hasn't seen him in months and is so glad they're together again. He suggests she joins the band in Boise for their next gig. She politely declines, saying she has to stay in Los Angeles because her show is two weeks away from its opening night. He points out it's a one-woman show and she can rehearse anywhere. She replies that the theater is set up for her in L.A. She notes that she is terrified that it won't go well, noting that while he doesn't seem to have concerns with performing, she has a lot of fear of whether people like her. She asks when the tour ends. He tells her there's no set amount of time but when it's done, they will then begin recording the record and then start another tour to promote the record -- the whole thing could be years. She asks him if he even likes the music they're doing. He is surprised by this and notes that the music is fine. Mia presses that he loves old school jazz. He doesn't understand why she's debating about his band she pressured him to make money to start a club and he went out and did that. She asks why he can't just begin focusing on the club with the money hes already earned. He tells her he's now part of something that is a success and that's what every musician strives for. Sebastian suggests she's surprised that the band would be successful because she suspected it would fail. He then adds that she probably doesn't want him to stay in the band to keep him down on her level (since she hasn't had much success as an actress). She asks if hes serious. He says yes. This moves Mia to tears and she leaves. Sebastian realizes the dinner has been burnt in the oven.
The day comes for Mia's one-woman show. Sebastian has promised to be there since he's in L.A. practicing with his band. As he heads out after practice, Keith reminds Sebastian of the photo shoot they have that night, for the album artwork. Sebastian didn't know it was that night and we realize he's going to miss Mia's show.
Mia performs her one-woman show across town, while Sebastian is photographed by an obnoxious photographer who asks him to bite his lip and lower his glasses to his nose. Mia's show ends and we see that only about ten people are in the audience. To add insult to injury, when she goes backstage, she can overhear people complaining that Mia isn't a very good actress and the show was bad.
Sebastian gets to the theater after it's closed but in time to run into Mia leaving. He apologizes for missing the show. She is upset and has lost faith in herself. She tells him her desire to be an actress was probably just a pipe dream and she's wasted six years, never realizing until now that she wasn't good enough to be an actress. He tries to encourage her but she points out she can't even pay back the theater because she had no ticket sales. She tells him she's got to regroup and is going home. He thinks she means her home in L.A. but she tells him she means home home, a.k.a. Nevada.
Mia drives back to Nevada and moves back in with her family. Inside her childhood home, Mia sees all the acting paraphernalia that decorated her room. Time goes by and Sebastian is in his apartment in L.A. He receives a phone call, someone asking for Mia. He tells her she is no longer accessible. The woman on the phone says she was told that this was the only number she could possibly reach her after she couldn't reach her through her home number. She tells Sebastian she is with Amy Brandt Casting.
In the middle of dinner at her parents' home, Mia is startled when someone outside lays on his car horn. Mia knows who it is immediately as this is how Sebastian would always get her attention. She goes to the window and see Sebastian across the street, with a neighbor yelling at him to be quiet.
Mia joins Sebastian outside. He tells her that a casting director who called him was one of the ten people who had seen Mia's one-woman show and now wants Mia to audition for a movie the next day at 5 p.m. Mia isn't sure if she wants to travel all the way back to L.A. to get her heart broken again. Sebastian tells her hell be outside her home at 8 a.m. if shes there, he'll drive her the five hours to Los Angeles; otherwise, he'll go back without her. She asks how he found her and he points out it's the house across from the library (she had mentioned this in the story about her aunt inspiring her to become an actress).
Sebastian returns the next morning and parks outside of her house but she's not there. He starts to drive away and then Mia steps into frame, telling him she was delayed because she got them coffee. She hops in the car and they drive to California. He takes her to the audition which now is just them in the waiting room, unlike her previous experiences. She is immediately called into the audition room, where both Amy Brandt and her assistant are friendly and happy to see her. Mia is told that the film will have a three-month rehearsal and then a four-month shoot in Paris. There is no script, as they say it will be built around the lead actress, so for her audition, they ask her to tell her a story. She thinks for a bit and then begins to tell them about how her aunt lived in Paris once. This monologue turns into a song, AUDITION (THE FOOLS WHO DREAM), about how Paris is the place for dreamers.
Mia and Sebastian go back to Griffith Park to catch up after their absence from each other. He asks how the audition went but she is afraid to be positive and get her hopes up. Mia asks Sebastian where they are now. He tells her Griffith Park but admits he's joking; he tells her he doesn't know where their relationship stands but he doesn't want to be the one to keep her from her dream. He is confident she will get the part and she has to devote all of herself to the opportunity, just like he did with his music. And that doesn't include him.
We see a palm tree and are told it's WINTER. But then we pull back to see it's actually a scrim with a palm tree and were on the Warner Bros. lot. We're told its FIVE YEARS LATER. Mia goes into the same coffee shop she used to work in. The new manager and employee are a bit star-struck and tell her the coffee's on the house. Like the movie star in the beginning of the film, Mia puts the money she intended to pay for the coffee with in the tip jar and then goes outside to a golf cart, where shes chauffeured to a set. Meanwhile, we see Sebastian in a small jazz club, testing out a recently tuned piano and talking with an employee about how business is.
Mia goes to an expensive home and greets her husband who is revealed to not be Sebastian but David (Tom Everett Scott). She is also shown to have a young daughter now. In another part of town, Sebastian arrives back at his home, where he is clearly better off than he was five years ago, but he does not appear to be living with anyone. He cooks lunch for himself alone.
Later that evening, a babysitter watches Mia and David's daughter, and they go out to an event, supporting a colleague in the industry. We also see Sebastian arriving at his jazz club, walking past a giant poster advertizing Mia's newest film on the side of his building. Mia and David get stuck in gridlock traffic with no cars moving, and Mia notes that this is what she doesn't miss about the city, letting us know she's bicoastal but based mostly in New York. She suggests that David exit the freeway and they go out to dinner instead.
After dinner, Mia is walking to their car but the sound of jazz music convinces David to scope out a nearby jazz club. He asks if Mia wants to check it out and she agrees. As they walk down into the club, Mia sees the name of the club on a neon sign - it says SEB'S. Inside, the club is packed with people as the band is finishing a song. Sebastian goes onstage to welcome the crowd and thank the band. Sebastian notices Mia in the crowd, temporarily losing his cool demeanor. He takes the spot of the pianist and begins to play MIA & SEBASTIAN'S THEME that he was playing when she first saw him. The lights dim on him and he plays, lit only by spotlight. When it's over, the lights come back up and we find him back in the restaurant where she first saw him playing the piano. Once again, Sebastian is fired by his manager (J.K. Simmons) for not sticking to Christmas tunes. He walks towards Mia who tries to compliment him but instead of brushing her off as before, he grabs her and kisses her.
This begins a lengthy epilogue, a ten-minute musical number where Mia and Sebastian dance from one set to another, symbolizing what their life could have been together. It starts with all of the restaurant's patrons and the manager dancing. Then we see them running through scenes of their life, scenes we've seen before, but a little bit better. They're at the jazz club with Keith approaching Sebastian but Sebastian turns him away before he can even say hello. They're at the theater where Mia performs her one-woman show, but the audience is full and they love it, and Sebastian is there cheering for her the loudest. They leave the theater and end up on an empty soundstage, and as they walk along it becomes the soundstage on the Warners Bros. lot with people dancing on a re-creation of the 105/110 interchange while a camera crew films. Amy Brandt appears to bring Mia in for her audition, and she and Sebastian embrace when she gets the part. They're both go to Paris with Mia filming a movie and Sebastian playing jazz. They end up getting married. Now we see them as a couple, with her returning home but it's a more humble abode than her movie star reality. Instead of David and her daughter, it's Sebastian and her son. They leave the baby with a babysitter and end up in the same gridlock traffic. Mia now suggests to Sebastian that they pull off the highway and go to dinner instead. She stops by their parked car but he leads her into the jazz club. They find the same seats that Mia and David found but now it's Sebastian next to her while they watch a pianist play on the stage. Sebastian and Mia seem closer and more affectionate than she does with David. But then the song ends and she's next to David with Sebastian onstage at the piano -- it was all a fantasy.
The crowd applauds, loving the song. David asks Mia if she wants to stick around for another. She contemplates the question but then finally says no. The two leave and she pauses at the door, looking back to make eye contact with Sebastian. He looks at her, and then very slightly smiles and nods his head. She smiles back at him, and turns to leave. Sebastian takes a moment, then begins playing with his band.