Captain Thorne Sherman and first mate Rook Griswold deliver supplies by boat to a group on a remote island. The group, consisting of scientist Marlowe Cragis, his research assistant Radford Baines, the scientist's daughter Ann, her recent fiancé Jerry Farrel, and a servant Mario, welcome the captain and his first mate, but subtly resist the visitors staying overnight, even though a hurricane approaches. Thorne goes with the group to their compound. Griswold stays with the boat, to come ashore later.
The viewer progressively understands that the situation in the compound is less than safe. During evening cocktails, Thorne becomes aware of a life-threatening situation. Marlowe Cragis performs well-meaning research on serums and uses shrews as test animals. The doctor's purpose is to make humans half-size in order to reduce world hunger; smaller humans will eat less food on a planet with a limited food supply. Unfortunately, the doctor's experiments created mutant giant shrews that escaped and are reproducing outside the compound, growing larger by the day. The scientist and his staff barricade themselves inside their compound each evening.
Thorne and Ann begin to fall in love, causing jealousy in Jerry. Meanwhile, outside the compound, the giant shrews, which have a poisonous bite, are running out of smaller animals to eat. The shrews (dogs with glued-on fur and screen shots of rapidly shaking, stuffed heads with large fangs) attack and kill Griswold. The giant shrews close in on the compound and break in through the basement. Thorne and Mario investigate the basement. Mario is bitten by the shrew, but Thorne shoots and kills it. The others arrive in the basement, but Mario dies. Radford discovers a highly toxic venom in the shrew saliva, the result of poison bait he had placed in an attempt to kill the shrews. Another shrew breaks in and kills Radford. From outside the compound, the shrews begin to chew through the compound walls on the main floor. The shrews
