In February 1969, Hollywood actor Rick Dalton, former star of 1950s Western television series Bounty Law, fears his career is on a downhill slope. Casting agent Marvin Schwarz advises him to make Spaghetti Westerns, which Dalton feels is beneath him. Dalton's friend and stunt double, Cliff Booth a war veteran who lives in a trailer with his pit bull, Brandy drives Dalton around town because Dalton's high-functioning alcoholism has resulted in too many DUIs. Booth struggles to find film work since the death of his wife, whom he is rumored to have murdered. Booth reassures Dalton that shooting spaghetti westerns can't be too bad.
Actress Sharon Tate and her husband, director Roman Polański, have moved into the house next door to Dalton's. Dalton dreams of befriending the couple to restore his status. That night, Tate and Polanski join Jay Sebring at a celebrity-filled party at the Playboy Mansion.
The next day, Booth repairs Dalton's TV antenna. He reminisces about a sparring contest he had with Bruce Lee on the The Green Hornet set, which resulted in Booth being fired on the spot. Charles Manson stops by the home of Polanski and Tate looking for record producer Terry Melcher, who used to live there, but is turned away by Sebring.
While driving Dalton's car, Booth picks up a young hitchhiker, "Pussycat". He drops her off at Spahn Ranch, where Booth once filmed Bounty Law. Booth is suspicious of the large number of hippies living on the property (the Manson Family), and suspects they are taking advantage of the owner, George Spahn. Booth insists on checking on Spahn despite objections from Lynette "Squeaky" Fromme. Spahn dismisses Booth's fears.
Returning to the car, Booth discovers that "Clem" Grogan has slashed a tire. Booth beats him and forces him to change it. One of the girls, "Sundance", fetches "Tex" Watson, but by the time Watson arrives on horseback, Booth is driving away. Meanwhile, Tate goes for a walk and decides to stop at a movie theater to watch herself in The Wrecking Crew, happy to see that the audience loves it.
Dalton lands the role of a villain in the pilot of a new series, Lancer, and strikes up a conversation with his precocious eight-year-old co-star, Trudi Fraser. During one scene, Dalton, hungover, struggles with his dialogue. After having a breakdown in his trailer, Dalton returns to the set and delivers a powerful performance that impresses Fraser and the director, Sam Wanamaker, bolstering Dalton's confidence. After watching Dalton's guest performance on an episode of The F.B.I., Schwarz books him to star as the lead of Sergio Corbucci's next Western, Nebraska Jim. Dalton takes Booth with him for a six-month stint in Europe, during which he appears in two additional Westerns and a European spy comedy, and marries Italian starlet Francesca Capucci.
Returning to Los Angeles, Dalton informs Booth he can no longer afford his services, and they agree to part ways. They go out for drinks and return to Dalton's home, where the jet-lagged Francesca is sleeping. Booth smokes an acid-laced cigarette and takes Brandy for a walk. Meanwhile, Tex, Susan "Sadie" Atkins, Linda "Flower Child" Kasabian, and Patricia "Katie" Krenwinkel park outside in preparation to murder everyone in Tate's house. Dalton hears their car and angrily orders them to leave. Tex drives their car down to the bottom of the hill and marvels at the fact that he just met his childhood hero. After Sadie reasons that Hollywood "taught them how to murder" and they should perform payback, they change their plans to kill Dalton.
Before the family members can proceed with their plans, however, Flower Child claims she left her knife in the car, so Tex hands her the keys. She instead drives off, deserting the other three. The others nonetheless continue and break into Dalton's house, confronting Booth, who recognizes them from Spahn Ranch, and waking up a frightened Francesca. Booth orders Brandy to attack, and together they're able to kill Katie and Tex and severely injure Sadie. Sadie, gun in hand, stumbles outside, alarming Dalton, who was lounging on a pool float and listening to music on his headphones, oblivious to the mayhem. She deliriously shoots the gun into the sky, so he retrieves a flamethrower that he still kept from a previous film he'd shot and incinerates Sadie. After the police come by to investigate, Booth is hospitalized for a stab wound he sustained in the confrontation and Francesca goes back to sleep, Sebring comes down the hill to ask Dalton what happened. Tate invites Dalton over for drinks with her house guests Sebring, Abigail Folger and Wojciech Frykowski.