Police Squad!

Season 1, Episode 6: Testimony of Evil (Dead Men Don't Laugh)

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A struggling comedian owes money to the owner of a nightclub. When his body is discovered at the bottom of a cliff in a car crash, all the clues point to suicide. It is later discovered that the comedian was also a police informant on a drug ring he infiltrated at his nightclub. Frank steps in and takes the place of the deceased at the nightclub in order to gather more clues.