Ed plays with bunnies and starts sneezing, itching, and exhibiting other strange symptoms, some of which are unrealistic. They wonder what's wrong with Ed, but it's evident that he's allergic to rabbits.

Lewis, a creative inventor, showcases his newest invention to Mr. and Mrs. Harrington. When a slight malfunction occurs, it creates a life-threatening situation for Mr. Harrington.

Panda and Clifford are having a fun time together when Clifford realizes Panda is using him to get close to his sister, Lucy. While running away from Panda, Clifford accidentally gets peanut butter all over himself, and he experiences a severe allergic reaction, causing his face to swell and his breathing to slow. Panda looks for the allergy pen to save Clifford and experiences a similar allergic reaction. Despite his condition, he manages to save Clifford, who in turn saves him with another allergy pen.

Chandler claims that he is allergic to dogs, but he really isn't. He finally admits that he's actually not comfortable with dogs.

An alien has taken control of the ship, and the crew need to fight it. Spock explains the situation in terms of biology: the ship is the body, the crew are the blood cells, and the alien is the virus.