"The Crowded Room" is reportedly inspired by the true story of Billy Milligan, who was arrested for raping three women in Ohio in 1975, as per Collider. Milligan was later diagnosed with multiple personality disorder, and became the first person to be acquitted on the basis of that disorder.
The show is an adaption of Daniel Keyes's 1981 novel "The Minds of Billy Milligan," which is based on the case, though the Apple TV+ series alters most of the details, changing Milligan's name to Danny Sullivan and changing his crime to a shooting. It follows Sullivan's interrogation, interspersing that with flashbacks to his upbringing.