Genesis

Picard and Data leave in a shuttle to recover a stray torpedo, leaving Data's pregnant cat Spot with Barclay. Barclay has a minor illness, and Dr Crusher treats him with a synthetic T-cell meant to activate a dormant gene. Soon the whole crew starts behaving strangely. Troi feels cold and constantly thirsty, Riker cannot think clearly and Worf turns aggressive. Picard and Data return days later to find the ship drifting and the crew de-evolved into earlier forms of life: Riker into a primitive hominid, Troi into an amphibian, Barclay into a spider. Data finds that the T-cell has mutated, become contagious and reactivated long-dormant genes. Picard is infected too and begins to regress. The pair find Spot changed into a lizard, though her kittens are unharmed, and Ogawa's unborn baby provides the key to a cure. Picard lures the savage Worf away from Troi, and Data's retrovirus restores everyone.