On a routine exploratory mission, the starship U.S.S. Hood picks up a distress signal from a research expedition though lost long ago, the expedition of Dr. Roger Korby, one of the centuries' greatest scientific minds. Korby himself is dead, it seems, but his colleagues have made a most incredible discovery, a discovery they insist the Hood's captain see for himself. Reluctantly, the captain agrees to beam down.
Meanwhile, the crew of the Enterprise begins long-overdue shore leave on Tranquility Seven. James T. Kirk is looking forward to a few days of rest and relaxation, until what seems like a bizarre case of mistaken identity plunges Kirk into a whirlpool of mayhem and murder, and puts an inhuman stranger with his memories and abilities in command of the Enterprise.