Chain of Command Part I & II (Star Trek: The Next Generation, Season 6, Episodes 10 and 11)
This is technically a two-part episode, but most fans agree you can start on Part II and just pay close attention to the "Previously on…" section. Picard, Worf, and Beverly Crusher investigate what they think is a Cardassian biological weapon. It turns out it was a trap, and Picard is physically and psychologically tortured and interrogated. When people reference the "There are FOUR lights!" episode, it's this one.
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