"The Cage" is virtually a lift from "Forbidden Planet" in almost every respect other than visual design.
Well, yeah ... but "Forbidden Planet" is lifted from "The Tempest", and Shakespeare used to rip off anything he could get his hands on. If the show works on its own merits, that's enough for me.
"Forbidden Planet" is based on "The Tempest" but in a very general sense - the resemblance between the play and FP is not nearly as direct as between FP and "The Cage."