Man from Atlantis

Imp (1x11)

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An impish little man enters an underwater station and causes the crew to become irrational simply by touching them. After three crewmen die the F.F.O.R. hears Duke, the last crewman talking, over the radio in a childish manner and investigates. Mark swims over and brings Duke over to the Cetacean, not knowing the imp has stowed aboard.