In the year 1959, a man named Felix (Nick Offerman) enters a hotel room and unrolls the carpet. He opens up the floorboards and stashes a bag full of money in there, and then puts everything back together like it was untouched. A while later, Felix is visited by a man. He invites the man into his room, only for him to shoot Felix in the back, killing him.
Ten years later.
Several individuals are arriving at the El Royale hotel in California. Father Daniel Flynn (Jeff Bridges) meets lounge singer Darlene Sweet (Cynthia Erivo), who is heading to perform in Reno. They meet vacuum salesman Laramie Seymour Sullivan (Jon Hamm), who is trying to fix himself a drink. He makes a few patronizing comments toward Darlene in regards to her race, though he claims to be joking. Moments later, the hotel's sole employee, Miles Miller (Lewis Pullman), emerges from the maintenance closet to tend to the patrons. A car then speeds up to the front, and in enters Emily Summerspring (Dakota Johnson), dressed like a hippie. The four sign their names (although Emily signs "fuck you") on the itinerary and then head to their rooms.
Sullivan is on the phone with his daughter to say the Lord's prayer while he is in the middle of checking every electronic device for wires. He goes outside his room and wanders through a hall that contains two-way mirrors into the other rooms. Sullivan sees Darlene practicing her singing while Flynn appears to be sitting silently. Sullivan is taken by surprise when he sees Emily pulling a young woman into the room and tying her up. Sullivan goes to a payphone to call his superior, and addressing himself as Agent Broadbank, stating that there is a situation at hand.
Darlene and her co-singers are practicing with their manager, Mr. Sunday (Xavier Dolan). He stops them in the middle of their song to speak to Darlene personally. Mr. Sunday criticizes her singing and talks down to her, saying that his time is far more valuable than hers, and that she should do what he says if she wants to make more money than she does now.
Flynn knocks on Darlene's door after hearing her sing, and he invites her to the lobby for a drink. From their conversation, we learn that Flynn is losing his memory as a result of possibly suffering some form of dementia. Flynn gets up to prepare the drinks, but he pours a drug into Darlene's drink. As he turns around to give her the drink, Darlene knocks Flynn out by smashing a bottle over his head.
We overhear a phone conversation between Sullivan and his superior. Their main operation is believed to have been compromised due to the wiring he found in his room, but Sullivan is preoccupied with what he saw in Emily's room. His superior orders him to stick to the original plan and to not interfere with whatever he saw.
On a beach, a man named Billy Lee (Chris Hemsworth) walks toward Rose (Cailee Spaeny), the young woman in Emily's room. He refers to her as "Boots" from her shoes, and he asks her why she looks so sad. It's implied that something has happened to one of her parents. Billy Lee takes his clothes off and invites Rose to go for a swim.
Back in the hotel, we learn that Rose is Emily's sister, and that she has taken Rose away from an apparently evil place. A flashback shows the girls as children when Emily tried to protect Rose from their abusive father. Sullivan knocks on their door because he is suspicious. Emily tells him to go away, so Sullivan knocks the door down and whacks Emily across the face. He goes to free Rose, but Emily rises with a shotgun aimed at Sullivan. Before he can explain himself, Emily shoots him dead. She then hears a groaning sound coming from behind the mirror she just shattered.
A flashback shows Flynn's life as a criminal prior to his incarceration for up to 15 years. In prison, he began to experience signs of dementia while his cell mate tried to calm him down.
After getting knocked out by Darlene, Flynn is found by Miles. Flynn shakes off his wound and asks for a drink. Moments later, Flynn and Miles go through the hall with two-way mirrors, and Flynn accuses Miles of spying on him. Miles insists that he didn't, but he says that the people he works for ordered him to spy on others. He mentions a man being there the previous year engaging in some illicit activity, and that there is a tape of him in the hotel that Flynn wants. As Flynn walks out, Miles notices the scene happening in Emily's room. Miles panics when he sees Emily wielding the shotgun. Flynn runs and tells him to get out of the way, but Miles is hit in the face by the buckshot, leaving him scarred.
We go back to Darlene's point-of-view following her knocking out Flynn. She retreated to her car when she noticed the scene with Emily and Sullivan. After Sullivan is shot, Darlene takes his gun and tries to get away in her car. Flynn knocks on the window and asks to step inside to explain himself, even as Darlene is holding the gun in his face. From another flashback, we see that Flynn and Felix were brothers who pulled off a successful heist. Felix hid the money in either Room Four or Five, and Flynn figures it was in Darlene's room. He only tried to drug her to get in her room and take the money without hurting her, but now that she knows, Flynn offers to split it with her if she helps him get it back. He also mentions the tape he is looking for and that it is valuable.
Emily brings Miles into the lobby all tied up. He knows she plans to kill him, though he tells her that what she did is not the worst thing he's seen in that hotel. He tells her how he saw a man write "sorry" in his own filth and that he had to clean up his corpse, among other horrible things he's witnessed. Rose calls someone on the phone, as she claims that it's not up to her or Emily to decide what Miles's fate will be.
Just as Flynn and Darlene are walking back from her room, they are both taken hostage by two people coming in with Billy Lee.
Another flashback shows that both Emily and Rose were members of Billy Lee's cult, where he presented himself as a charismatic leader. Rose fell in love with him and it appeared that he felt strongly for her too, as he still called her "Boots" affectionately. He got Rose to fight another girl to compete for a chance to keep him company in his room, and Rose viciously attacked the other girl so that she could win.
In the hotel, Billy Lee has two of his cronies tie up Flynn, Darlene, and Emily. He starts to spew some phony philosophical nonsense as he notices the money bag. Darlene claims it is hers, but then soon confesses Flynn's part in taking the money. Billy Lee also inquires about the tape that Flynn found and why it is of such great value (it's implied to be of RFK). Meanwhile, Miles tries to ask Flynn to absolve him of his sins, still believing Flynn is a priest due to how he's dressed. Flynn admits that he only dressed that way so that nobody would ask him questions. Billy Lee then decides to play with his hostages' lives with a Roulette wheel. He makes Emily be red while Miles picks black after being pressed to do so. They land on black, so Billy Lee shoots Emily in the heart. With her dying breaths, she looks at Rose, who doesn't seem too broken that her sister has just been killed. Billy Lee then orders Darlene to sing so that he can determine whether or not she should live. After some hesitation, Darlene starts to sing, but Billy Lee spins the wheel anyway, because he claims he's heard better. Flynn rises and headbutts Billy Lee. His cult members try to shoot Flynn, but Darlene intervenes to stop them. Flynn and Billy Lee fight and knock over something containing fire, which starts to spread around the lobby. Darlene and Miles hide in the chaos as Sullivan's gun lands at Miles's feet. Darlene tells Miles to pick it up, but Miles says he cannot kill anymore. Darlene asks how many people he's killed, and he says over 100.
We see Miles as a young boy learning to shoot a rifle, up to his later years fighting in Vietnam. He walks among a field of dead soldiers, horrified at what he has witnessed, as well as what he has participated in.
Back in the lobby, Darlene tells Miles it's okay if he chooses not to kill. However, Miles gathers his strength and takes the gun. He shoots Billy Lee in the head and then kills his cult members, along with two other members who were waiting outside. Rose cries over Billy Lee's body as Miles goes to apologize and comfort her. She grabs a knife and stabs Miles in the gut, and she is then shot dead by Flynn. Flynn and Darlene try to help Miles, but Flynn knows his wound is too severe to fix. While still pretending to be a priest, Flynn absolves Miles of his sins before he dies. Flynn and Darlene gather the money as the hotel continues to burn down. Flynn also gives Darlene the tape for her to toss into the fire before they leave the El Royale for good.
A while later, Flynn is dressed in a fancy white suit as he walks through a casino. He goes to see Darlene as she is about to perform a song. As she thanks the audience for attending, she notices Flynn in the crowd and she smiles before she starts to sing.