Who killed Marsha P. Johnson? In July 1992, Johnson’s body was found floating in the Hudson River near New York City’s Christopher Street Piers. The police deemed her death a suicide, but for those who knew her, this theory has never made sense.

Johnson — a veteran of the Stonewall riots; a co-founder, along with her friend and comrade Sylvia Rivera, of Street Transvestites Action Revolutionaries (S.T.A.R.) and the modern trans rights movement; a self-described “street queen”; a sex worker; and a New York celebrity — would never, according to her loved ones, have harmed herself. Twenty-five years later, Victoria Cruz of the Anti-Violence Project has amassed a pile of evidence of foul play.

But this absorbing film by Academy Award–nominated filmmaker David France (How to Survive a Plague) is much more than a search for answers about Johnson’s death. With riveting archival footage and interviews with the people who knew, loved, and fought beside them, it’s also a celebration of the lives and legacy of both Johnson and Rivera. Together, they blazed a path for gender-nonconforming people and demanded that they be included in the fight for civil rights (demands that were often met with scorn by gays and lesbians in the 1970s).

France follows Cruz on her quest for answers, while using archival footage of Johnson and Rivera (who died in 2002, of liver cancer) to paint vibrant portraits. At the same time, the film provides a somber reminder of the current epidemic of violence against trans women: as Cruz researches Johnson’s death, her organization is fighting for justice for Islan Nettles, a transgender woman murdered in 2013.