The Cube (1969)

An unnamed man, simply called "The Man" is trapped in a cubical white room where anyone else can enter and leave, but which he himself apparently cannot leave. A stool is brought in covered in strawberry jam, the furniture changes throughout the play. The main character, is subjected to an increasingly puzzling and frustrating series of encounters, as a variety of people come through various hidden doors. But, as many remind him, he can only leave through his own door, so he must find it to leave.