Star Trek: The Original Series, season 1, episode 27
The Alternative Factor
Summary
While orbiting an apparently dead planet, the Enterprise seems to experience a strange moment of "nonexistence". Captain Kirk discovers a man named Lazarus on the planet below, who claims the effect was caused by his "enemy", later revealed to be an insane version of Lazarus from an alternate dimension. The sane version of Lazarus asks for Kirk's help in defeating his counterpart.
Summary from Wikipedia, CC BY-SA 4.0.
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