Quentin Tarantino's Once Upon a Time in Hollywood is based on real events, but with an important caveat: it is an alternative history. The plot centers on Hollywood in the late 1960s and the murder of actress Sharon Tate, the wife of director Roman Polanski.
In reality, she was brutally murdered by members of the Charles Manson cult in 1969. In the film, these events receive a fictional ending: the main characters, actor Rick Dalton and his stunt double Cliff Booth, accidentally end up in the path of the killers and brutally kill them, saving Tate.
Many of the supporting characters are real historical figures: Bruce Lee, Steve McQueen, Jay Sebring, George Spahn. The film also mentions real TV shows and films, and Spahn's ranch, where Manson's followers lived, actually existed.

The film thus combines documentary details with fiction, imagining how things might have been if history had gone differently.