TOS #27: The Alternative Factor

While an investigating an uninhabited planet, the ship gets rocked by this strange force that makes things' gravity go momentarily near 0. The Captain, Spock, and several redshirts then beam down to talk to a newly discovered, single, humanoid life form on the planet, who they bring back to sick bay on the Enterprise.



This guy turns out to be schizophrenic, and one of his personalities is a dilithium crystal kleptomaniac. Eventually, they figure out that there is a singularity connecting parallel universes, and that this dude is actually jumping back and forth. The other him, from the parallel universe, is trying to stop him from accidentally annihilating everything.

Eventually, one of his selves and Kirk manage to lock both halves of the dude in this interstitial space, protecting both universes but dooming both incarnations of the guy to eternal hand-to-hand combat in a very blue room.