Stardate 47653.2: When Data and Picard return from retrieving a wayward demonstration torpedo, they find the ship without control and the crew mutated to what seems to be lower lifeforms. Deanna is an amphibious creature, Riker an Australopithecine. While Picard is being chased by Worf, who has turned into a massive proto-Klingon predator, Data discovers that a synthetic T-cell that Dr. Crusher has given to Barclay to fight a flu has unlocked not just the one gene that Barclay needed. The T-cell became airborne and activated the crew's dormant introns, genes of previous, more primitive lifeforms. Barclay himself was turned into a spider. Only the new-born kittens of Spot, Data's cat, are immune, while Spot herself is now an iguana. With the help of pregnant Nurse Ogawa, who is now a primate, Data is able to find a remedy to reverse the de-evolution in the form of a retrovirus.

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