The given stardate is 47653.2, the year 2370. Worf fires a torpedo that veers off course during weapons exercises, and Picard and Data leave in a shuttlecraft in order to retrieve it from space. Data leaves his pregnant cat, Spot, with Barclay. Barclay, a hypochondriac, visits Crusher, who gives him a synthetic T-cell to activate one of his dormant genes. However it has the undesired side-effect of activating all his dormant introns. The crew begin to experience strange symptoms. Worf becomes aggressive and feels hot, while Deanna Troi feels cold. Crusher examines a silent Worf in sick bay and notices a venom sac on his neck, whereupon Worf sprays venom (it is not clear if this is inadvertently or deliberately) at Crusher, injuring her severely. As a result of her injuries, Crusher is placed into stasis.
Several days later, when they have retrieved the torpedo from space, Picard and Data return to find the ship adrift, with the main power off-line. One of the bridge officers is dead, torn open in his seat, and most of the crew are de-evolving. Worf has reverted into an aggressive predator attempting to mate with Troi; Riker a caveman; Troi an amphibian; and Barclay a spider. Picard is overcome by irrational emotions of fear and anxiety, indicating that he has been infected, and Data suggests he might soon de-evolve into a primate similar to a lemur or marmoset. Picard and Data return to Data's quarters and find Spot and her kittens. Spot herself has changed into an iguana, however the kittens are normal. Data recommendeds that they locate Nurse Alyssa Ogawa who is recently pregnant. While in sickbay, Worf attempts break through the doors searching for Troi. Data concocts a pheromone spray from one of Troi's sebaceous glands, and Picard uses it to distract Worf and lead him off through the ship, so Data can work on the cure. Worf eventually catches Picard in a narrow crawlspace and the Captain uses a power cable to incapacitate him. Meanwhile, Data finishes the construction of the modified retro-virus and releases it into the air which returns everyone to their original state. At the conclusion, Crusher names the condition 'Barclay's Protomorphosis Syndrome', after its first sufferer.