In the opening shot, a helicopter carries a giant, plastic hot dog over sunny Los Angeles, California, and sunbathers wave to it from a pool overlooking the city. Television weatherman Harris K. Telemacher (Steve Martin) avoids traffic on the way to work by cutting through backyards, parking lots, and alleyways. He arrives at the KYOY-Chanel 14 television station, dons a safari hat and performs his "wacky" weather report. Afterward, Harris's boss tells him to be "more wacky, less egghead."
Harris goes to pick up his girl friend, Trudi (Marilu Henner), on the way to a lunch date with eight other people, and they argue about punctuality. He complains that Trudi takes too long to get ready, but Trudi urges him to relax. They finally get into Harris' car, and Trudi says that she invited Harris's agent, Frank Swan, whom Harris wants to fire. A radio deejay announces that it is the first day of spring and Harris panics, because the first day of spring is "open season" on Los Angeles freeways. He tells Trudi to grab his gun out of the glove compartment. She loads the weapon and hands it to him, and he shoots at other armed drivers.
The couple shows up late at the restaurant, but are still the first to arrive. They are eventually joined by talent agent Frank Swan (Kevin Pollak) and Roland Mackey (Richard E. Grant), a British art dealer. Roland's ex-wife, Sara McDowel (Victor Tennant), shows up even later. She introduces herself as a journalist writing an article about Los Angeles for the London Times. An earthquake interrupts, and everyone but Sara ignores it. Harris stares at her from across the table, and Trudi becomes jealous. Leaving the restaurant, Roland tries to rekindle his romance with Sara, but she says she likes their relationship as it is.
At a clothing store, Harris tries on a pair of white slacks while a bouncy, dimwitted shop girl named Sandee (Sarah Jessica Parker) marks them for tailoring.
That evening, as Harris and Trudi drive home, Harris's car slows to a stop on the highway. He pulls over to look under the hood, and a passing breeze prompts him to turn around. He sees "Hiya" written on an electronic freeway sign. The magical electronic sign communicates with Harris in writing, asking if he is okay and requesting a hug. The sign says it stops people who are in trouble. Harris hears his engine restart as the sign reads: "You will know what to do when you unscramble how daddy is doing." Harris promises to work on the riddle and waves goodbye.
Soon after, Sara sees Harris's wacky weather report on television and telephones, asking to interview him for her article. Harris and his best friend, Ariel (Susan Forristal), go to the Los Angeles County Museum of Art, where Ariel records Harris with a video camera as he roller-skates through the galleries.
Later, Sara and Harris meet for the interview. He takes her on a driving tour of the city, including a stop at a cemetery. When a gravedigger digs up the skull of The Great Blunderman, a magician, Harris says he knew the man. Sara impresses Harris with a fitting quote from William Shakespeare's Hamlet. He asks if she is seeing anyone, and she says yes. Later, at the KYOY-14 station, a producer asks Harry to do the weather report during the weekend, but Harris insists on pre-taping, promising that the weather will not change. A surprise rainstorm ensues.
When Harris goes to pick up his altered slacks, the cute shop girl, Sandee, apologizes that the pants are not ready and takes his phone number. On the drive home, Harris sees the talking freeway sign. It tells him he should have gotten Sandee's number. At home, as Harris laments the missed opportunity, Sandee calls to ask him out. They go to a restaurant, where Sandee points out her boyfriend at the bar, and explains that they have an open relationship. She says her name is spelled "SanDeE" and draws it on Harris's hand. Elsewhere, Roland and Sara eat hot dogs and watch skateboarders. Roland kisses Sara, but she is unmoved.
The next day, Harris goes to Trudi's apartment, who reveals that she has been having an affair with Frank Swan for three years. Relieved, Harris does a celebratory dance outside. He tells the freeway sign that he is now free from his relationship and his agent. The sign responds that the weather will change Harris's life twice. Soon after, Harris is fired because his pre-taped weather report failed to predict rain.
Sandee urges Harris to get a colonic irrigation and they go to the clinic together. They make plans for dinner, but Harris cannot get a reservation at L'Idiot, the hip, new restaurant Sandee suggested, so he makes plans for a weekend getaway instead. Meanwhile, Sara records her observations about Los Angeles on a Dictaphone, observing that it is a place where people have taken a desert and turned it into their dreams. She also notes that it is a place of secrets, and disagrees with Roland, who believes Los Angeles is a place for the "brain dead."
At a museum, Roland and Sara peruse art while Ariel records more video of Harris on his roller-skates. Harris accidentally runs into Sara, who invites him to join her and Roland for dinner. They go to L'Idiot, and afterward, Sara gives him a ride home. Arriving at his apartment, they step out of Sara's car, and it begins to move on its own. Sara panics, but Harris says he knows what is happening. They jump back into the car, and it leads them to the freeway sign. The sign tells Harris to kiss Sara, and he does. The sign then urges him to let his mind go, promising that his body will follow. In a daze, Sara agrees to go to a fundraiser with Harris the following evening. Later, she telephones her mother and plays "Do Wah Diddy Diddy" on the tuba, while her mom accompanies her on piano.
The next night, as he arrives to pick her up, Harris overhears Sara playing the tuba. He detects Sara's nervousness and encourages her to relax and give in to the passion she feels. She says he should do the same, and suggests he uses silly antics to avoid getting hurt. At the fund raising dinner, Harris pretends to be ill and lures Sara outside. They kiss, and he tells her to let her mind and body go. They return to the table disheveled, just as the long-winded host finishes his speech.
The next day, Harris tells Sandee he cannot go away with her that weekend because he is seeing someone else. When he admits that Sara is going on a trip with her ex-husband, Sandee convinces him it is only fair that they have a vacation, too. Harris and Sandee unwittingly end up at the same Santa Barbara resort as Sara and Roland. Unaware that Harris is next door, Roland and Sara overhear him and Sandee having sex and are inspired to do the same. Afterward, Harris and Sara are dismayed when they bump into each other in the hallway. Blissfully ignorant, Roland suggests they have dinner together.
The couples walk on the beach, and Sara and Harris steal time alone. Harris begs her to stay in Los Angeles, saying he wants to get married, have children, and grow old together. For Sara, however, their fling is everything she didn't want: pain, lying, and complications. Checking out of the hotel, Roland runs into Harris and gives him the sad news that Sara has decided to go back to London as soon as possible.
Back in Los Angeles, Harris finds Sara packing at her apartment. He begs her to reconsider, but she refuses. At his new job as a news anchorman, Harris mopes, while Sara heads for the airport. Another rainstorm hits, and she changes her mind. She goes to Harris's apartment, and when he sees her, he notes the rain, and says, "That's twice," in reference to the freeway sign's suggestion that the weather would change his life twice.
Harris takes Sara back to the freeway sign, which tells him to unscramble "How Daddy is Doing." Sara figures out the riddle by unscrambling the phrase, and says the sign is telling Harris to "sing Doo Wah Diddy." Harris asks the sign why it chose him, and it says, "There are more things 'n heaven and earth, Harris, than are dreamed of 'n your philosophy," then returns to reporting the traffic. Harris and Sara kiss beside the road and Harris closes the film with a final voice-over narration which he states that it is sometime in the future and that there is someone for everyone, and he will never forget the night when he learned that romance does exist deep in the heart of Los Angeles.
The final shot show the magical traffic sign displaying the text: "What I really want to do is direct."