'The Cube' is about the existential dilemma faced by a man who suddenly finds himself trapped inside an 8 foot by 8 foot cube. He can't get out, but apparently other people can come and go as they please through panels in the walls. This philosophical teleplay covers many themes such as, the nature of reality, individual versus scientific perception, self-reference, man’s relation to others and society, insanity, social projections, and race relations.

Produced and directed by Jim Henson, it 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).

This information cut together from here and here. Apparently there's also an entire Yahoo! Group dedicated to this teleplay.