Cookie Chomper III was Alvin, Simon and Theodore's first pet, a stray kitten that finds its way into the Seville house. The boys help Dave overcome his allergy to cats and Cookie Chomper is allowed to stay. However soon Cookie Chomper is hit by a car and killed.
Dave helps the boys deal with their grief and embrace the memories of their brief but happy time with Cookie Chomper III, and later they adopt Lilly.