Based on or true?

Mostly fictional

It depends. It basically is based around the true story of Charles Manson and his "family" and the murders that took place. Within the Manson specific plotline there are definite truths that happened (Dennis Wilson's involvement in knowing him plus some the same and some name changed 'family' members aka: the girls and their involvement).

Though the story of the characters outside of that are all fictional *though* stories about the Black Panthers, dodging or defecting from Vietnam, the police department and true treatment of women. Not to mention societies treatment of black/white relationships and segregation is all fact. They just built fictional characters around real things that happened in that time.