Captain Jean-Luc Picard and Data return to the USS Enterprise to discover the rest of the crew de-evolved into primitive forms of life, including spiders, amphibians and cave men.

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 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, and William Riker has difficulty concentrating. Crusher examines a silent Worf in sick bay and notices a venom sac on his neck, whereupon Worf sprays venom in Crusher's face, injuring her severely. As a result of her injuries, Crusher is placed into stasis. Nurse Ogawa warns the senior staff that everyone on the ship is in danger.

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