The Cube

Description from Wikipedia:

The Cube was an hour long teleplay that aired on NBC's weekly anthology television show NBC Experiment in Television February 23, 1969. The production was produced and directed by puppeteer Jim Henson, and was one of several experiments with the live-action film medium which he conducted in the 1960s, before focusing entirely on the Muppets and other puppet works. The screenplay was co-written by long-time Muppet writer Jerry Juhl (who also appears in a cameo).

The teleplay starred Richard Schaal as a man trapped in a cubical white room that anyone else could enter and leave but which he himself apparently could not leave. The main character, simply named "The Man", 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.

At one point Henson comments on his own teleplay through a Professor who wanders in from yet another door.

PROFESSOR: Well, as I interpret what you’re doing here, this is all a very complex discussion of Reality versus Illusion. The perfect subject for the television medium!

MAN: What do you mean, television?

PROFESSOR: Well, this is a television play.

MAN: What?

PROFESSOR: Oh, you don’t believe that?

MAN: Of course not!

PROFESSOR: I should have thought you’d want to. After all, there’s only one other possible explanation.

MAN: Which is?

PROFESSOR: Hallucination. That you are altogether insane.

The teleplay only aired twice (it was rerun in 1970).

The central plot point is strikingly similar to "The Squirrel Cage", a short story by Thomas M. Disch that was published in 1967. Both stories are about a man who is imprisoned in a big white cube. The man doesn't know why he's there and never finds out. He never gets out of the cube. Disch's story also appeared in his collection Fun With Your New Head (Doubleday, 1968).

In 2007, the German Fringe Theater troupe GlassBooth, Directed by Roger Hoffmann and starring Jens Dornheim presented a live stage adaptation of "The Cube" titled "KUBUS", which performed to packed houses and critical acclaim.