On his way home from work, a flu-stricken Maxwell meets Dr. Jekyll and Mr. Hyde. Delighted, Maxwell decides to put up Dr. Jekyll in exchange for his medical services. At first, the family finds it handy to have a doctor hang around the house. His medicated shampoo promises to help restore sheen to Brady's sister Arlene’s hair. Pollyanna, Brady's mother, is given a special powder to give her a radiant complexion. But his cosmetic concoctions have undesired, freakish side effects. Brady is very suspicious about the doctor’s orders. Brady realizes the doctor’s fiendish plan: the Plunkett family is slowly turned into mutants and is eventually sold to the local pet shop. While Brady attempts a daring monster rescue, his twin brother and sister, Marvin and Gaye, work on a reverse transformation formula.