Star Trek: The Original Series, season 1, episode 14
Balance of Terror
Summary
While investigating a series of destroyed outposts, the Enterprise discovers a lone Romulan vessel with a cloaking device. The Romulans, having never been seen by humans, are revealed to visually resemble Vulcans, casting doubt on Mr. Spock's loyalty, as the two ships become locked in a cat-and-mouse battle through space.
Summary from Wikipedia, CC BY-SA 4.0.
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