A virus turns the crew into earlier life forms.
Dr. Crusher gives the crew a treatment for a minor illness, and soon the crew begins to change. Members of the crew regress to earlier evolutionary forms, including a spider and a primitive human. Data, being an android, is unaffected, and he has to find the cause and a cure.
The story is a piece of science-fiction horror and comedy, with creature effects and makeup. The crew has to find a solution before the ship is lost. It has a reputation as one of the odd stories of the season, and it is a fun, strange adventure.