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In 1977, after Billy Milligan is accused of multiple rapes near Ohio State University, experts question if he has multiple personality disorder.

Billy's troubled youth comes into focus via interviews with his siblings and childhood friends. At trial, his lawyers mount an unusual defense.

As Billy's celebrity rises, a new doctor disagrees with his diagnosis and alters his treatment. Later, Billy takes his care into his own hands.

With the help of friends and family, Billy goes on the run, changing his identity and mixing with some of Hollywood's biggest names.