A play by Peter Weiss set in post-revolutionary France in the insane asylum at Charenton. Its full name is 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, which is why nobody ever uses it.
A little background: the Marquis de Sade lived out his final years in this very asylum, confined there by his political enemies. To amuse himself, he would write plays and have the other inmates perform them.
Marat/Sade begins with the idea that de Sade wrote a play about the assasination of Marat, and that the inmates are performing this play (so Marat/Sade is also a meta-play). The play-within-a-play gradually deteriorates until...but that would be telling.
This is the only musical I like.