Identify this play that was on PBS in the very early '70's

When I was a young child, I watched this play on PBS. This was pre-cable and it was a boring, lazy weekend afternoon with nothing to do. So, even though there was no way I’d understand it, I watched this really weird show about a guy in some sort of prison cell. Here’s what I remember:

[li]White looking walls with black looking lines to simulate the appearance of tiles;[/li][li]A cook bringing food in and this seeming to be an opportunity to escape - but it’s not;[/li][li]A harpy of a mother visiting the prisoner;[/li][li]Some crazy seeming younger female visitor who leaves the cell via a wall that disappears revealing a busy city street.[/li][/ul]

Google reveals nothing. So, I appeal to my fellow Dopers, one of whom revealed the name of the movie I’d always known as “Murderer, Murderer” to be “The Black Menace” in one post: what was this bizarre play, movie, show or whatever?

Sounds like it could be The Cube. Jim Henson (yes, him) wrote and directed it, and it was first broadcast on NBC’s Experiment in Television (which would explain why you might think it was on PBS – Experiment in Television was shown commercial free).

Here’s a Wikipedia article on The Cube. Your memory is pretty impressive, Caricci – this was apparently only played twice, once in 1969 and once in '70! But considering how creepy it looks, I can imagine this staying with you as a young kid.

How much do I love the Dope! I am just floored. Seriously, you guys are great! You know, I was in kindergarten when I watched this, or maybe first grade. I am almost 47 now.

Wow! First Murderer, Murderer and now this. Honestly, I am just thrilled. Now I’ll go read the Wikipedia article and watch the video.

ETA - yeah, creepy.

And furthermore, I was right about it being on on a weekend afternoon.