Doctor Walter Frobish, a scientist working on his laboratory on an island, discovers with the help of his daughter Phoebe a way to electrically stimulate lobsters, in order to grow their size. One night, a person breaks into the laboratory and injects dangerous steroids into lobsters. The electrical stimulation combined with this injection causes unexpected effects, and the lobsters become huge killing machines that eventually escape into the ocean. The first strange deaths begin to appear along the coasts…