Stardate 3087.6: While the Enterprise is in orbit around an uncharted planet, the ship as well as regions throughout the galaxy are struck by a spatial anomaly that emanates from the planet. Starfleet Command suspects a pending invasion and orders Kirk to investigate the case. On the surface a landing party finds an obviously paranoid humanoid called Lazarus, who claims to chase an evil "thing" disguised as another humanoid that is out to destroy the universe. Once aboard the ship, Lazarus switches places with a doppelganger because of another anomaly, against his wishes and unbeknownst to the crew. This other person steals dilithium crystals from engineering and vanishes, leaving behind the already known Lazarus. This insane Lazarus too gets his hands on the ship's dilithium crystals and beams down to his ship. When Kirk follows him, he steps into the ship and is promptly transferred to a parallel universe. Here the other (sane) Lazarus, the "thing", is preparing another ship of the same type, using the dilithium crystals he took. He tells Kirk that the mad Lazarus must be stopped, because if the two ever met on one side of the safe corridor between the two universes, it would destroy everything that exists. So he sends back Kirk to our universe and prepares to enter the corridor one last time. Once he is back, Kirk pushes the mad Lazarus through the gate of his ship, and orders its destruction. The two are trapped in the corridor for all eternity, which saves both universes.

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