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Charter boat owner Thorne Sherman and his first mate Rook arrive at a remote island bearing supplies for its inhabitants, biologist Dr. Marlowe Craigis, his daughter Ann, a zoologist, two researchers, Jerry Farrell and Radford Baines, and Mario, their cook. Ann and Craigis are inexplicably disappointed when Thorne announces that he intends to ride out a threatening hurricane on the island rather than return to the mainland immediately. Leaving Rook behind to anchor the boat, Thorne accompanies the Craigises back to their rough-hewn house surrounded by a high wooden fence and asks why the scientists are unaware of the storm warnings. Farrell acknowledges their radio has been broken for more than a week, but offers no further explanation. After Craigis describes the scientists' interest in overpopulation that guides their study of genetics and heredity, the avid Baines displays one of the test subjects, a tiny Blarina shrew, and details the scientists' attempts to manipulate its genetic makeup. When Craigis indicates that they have produced a variety of the creature that must eat three times its weight to survive, Ann grows increasingly uneasy. Meanwhile, Rook rows a dinghy to shore and lashes the boat to a tree as the wind begins blowing harder, unaware that he is being stalked by a pack of strange animals. Upon seeing the unusual creatures, Rook races into the woods and climbs a tree to evade them but his cries for help go unheard as the wind and storm rise. Unsettled by the growing tension in the Craigis house, Thorne declares he must return to the boat and is shocked when Ann, brandishing a pistol, refuses to let him depart. After sheepishly putting down the gun, Ann admits that their experiments have gone awry, allowing one of the test shrew groups to mutate into giants weighing up to a hundred pounds. Due to an inadvertent error by Farrell, several of the mutants escaped into the woods and, having rapidly multiplied, are mad with hunger. Farrell adds that the unaltered specimens are nocturnal but the mutants appear to be unpredictable as he and Ann were nearly attacked the day before, prompting Craigis to insist on Ann's immediate departure from the island. Unknown to the group, the shrews have attacked and devoured Rook and in a ravenous fury have begun digging under the compound's wooden fence. When a few shrews get into the livestock pen and attack the animals, Thorne demands to return to the boat but is restrained by Craigis and Farrell. Although frustrated, Thorne agrees to remain in the compound all night but insists they must go to the boat the next morning. In the meantime, he suggests that the group set up a watch to insure none of the shrews break into the house. Later that night as Mario patrols the house, the wind blows open one of the shutters in the back of the house and a shrew squeezes inside through the window. Upon finding the open shutter, Mario ties it shut, then wakens Thorne to inform him a creature has gone to the cellar to attack the food supply. Armed with pistols, Thorne and Mario descend to the cellar, where the cook is immediately attacked by the shrew. Although both men fire and kill the creature, Mario sustains a large bite on his leg. Attracted by the commotion, Craigis and Farrell join Thorne, who quickly wraps the wound, but moments later the men are shocked to find that Mario has died. Baines and Craigis examine the dead shrew and Mario's wound, and discover that a high amount of poison is present in the creature's saliva. Craigis recalls having set out a large store of rat poison some weeks previous in an attempt to kill the shrews and the scientists conclude that the beasts adapted the poison into their system. As the group considers their next action, two shrews chew through the soft adobe walls to the house and enter a back bedroom. Toward dawn, Thorne suggests that they throw the carcass of the dead shrew outside to draw off the pack, allowing them to get to the boat. When the ploy fails to attract any shrews, Thorne insists on making a run for the boat and Ann demands that Farrell accompany him. In the woods, resentful of the attention Ann has shown Thorne, Farrell threatens him with a shotgun, but Thorne knocks Farrell down and takes the gun away. Close to shore, Thorne calls to Rook on the boat and is puzzled to receive no response until he finds shreds of Rook's clothing and an empty pistol. Upon hearing the shrews approaching, Farrell panics, runs back to the compound and locks the gate, refusing to allow Craigis to open it for Thorne. As the shrews chase Thorne to the fence, he scrambles over the top, then beats up Farrell in a rage. After the men calm down, Ann heads toward the kitchen but when she opens the door a shrew rushes out and grazes Baines before it can be shot. Baines insists he is unhurt and returns to writing reports, only to drop dead moments later. Craigis admires that the scientist was bravely chronicling his symptoms as he died. When the others realize there are several more shrews in the house, they retreat to the courtyard and, at Thorne's suggestion, begin examining the supplies for assistance in combating the creatures. Thorne recommends lashing together the large, empty, steel chemical drums to use as shields to allow them to make their way to the boat, but the hysterical Farrell refuses and seeks refuge on the roof. The others pull the heavy drums over themselves as shields and, carefully opening the compound door, slowly proceed outside. Ann is soon exhausted from carrying the heavy drum, forcing the group to rest several times as the shrews desperately attack the containers. Farrell watches from the rooftop and when the shrews all congregate at the drums he makes a wild dash through the woods. The shrews hasten after him, however, and he is attacked and killed. After a laborious effort, the others reach the water, which the shrews will not enter, and discarding the drums, swim safely to the boat.