The Alternative Factor

A sudden disturbance makes everything in the region briefly wink out of existence, and the Enterprise finds a lone man, Lazarus, on a barren planet below. He claims he is chasing the creature that destroyed his civilisation, but his story keeps changing and his injuries come and go. The truth is that there are two Lazaruses, one from our universe and one from a parallel universe of antimatter, linked by a corridor between them. One is a raving fanatic, the other rational. If they meet outside the corridor, both universes will be destroyed. The mad Lazarus steals the Enterprise's dilithium crystals to pursue his double. Kirk forces him into the corridor, where the sane Lazarus holds him, and the Enterprise destroys the ship that acts as the gateway, trapping the two men in endless struggle forever.