"The Persecution and Assassination of Jean-Paul Marat as Performed by the Inmates of the Asylum at Charenton Under the Direction of the Marquis de Sade" is a play written by Peter Weiss, whose conceit is that a group of inmates in a post-revolutionary French insane asylum are talked into acting out a subversive denunciation of the revolution's betrayal by the Marquis de Sade, who is a co-inmate. And it's a musical.
My parents loved this one, and we sang the songs in the car. I've seen several productions, including the 1967 film of a live performance. It turns out that this whole film is online on YouTube, all two hours' worth. It's quite an amazing work, and the YouTube version is subtitled in English, Portuguese (Brazilian), French, Spanish and Turkish.
The Marquis de Sade is locked in the Charenton mental hospital and decides to put on a play. His overseers agree as long as he follows certain conditions. He writes and directs the other mental patients in a play based on the life of the Jean-Paul Marat. As the play progresses, the inmates become more and more possessed by the violence of the play and become extremely difficult to control. Finally, all chaos breaks loose.