Captain Thorne Sherman [James Best] and his first mate [Rook Griswold] deliver supplies by ship to a research compound on a remote island as an intense hurricane gathers offshore. The station’s residents—Dr. Marlowe Craigis [Baruch Lumet], his research assistant Dr. Radford Baines [Gordon McLendon], Marlowe’s daughter Ann [Ingrid Goude], her fiancé Jerry Farrell [Ken Curtis], and a servant Mario [Alfredo de Soto]—offer a cold, wary welcome and insist they unload quickly and depart before the storm breaks. Thorne refuses to abandon the locals to the treacherous weather, arguing the coast guard won’t reach them in time, and he decides to stay while Rook remains with the boat.
The core obsession at the heart of the tale is Marlowe’s attempt to solve a global problem by shrinking humans. He has been chasing the dream of isolating the genes responsible for growth and metabolism, aiming to reduce people to half their size to ease the burden of overpopulation. He tests his ideas on shrews, chosen for their short lifespans and rapid generational turnover, so he can observe long-term results in a relatively brief period. The atmosphere in the compound grows tense as Marlowe explains his method and the consequences he fears if the experiments ever escaped the lab.
As evening settles, Thorne and Ann converse, and the mood tilts toward personal strain. Jerry, feeling left out and defensive, confronts Ann about their engagement, and she bluntly ends things, citing his earlier cowardice. Thorne tries to retreat to his boat to escape the awkwardness, but Ann stops him, pulling a gun and insisting he stay. She reveals a troubling truth: Marlowe’s experiments have produced a batch of mutant, wolf-sized shrews that escaped due to Jerry’s drunken negligence, and these creatures are now reproducing in the wild. To keep themselves safe, the group barricades the compound every night, slipping the heavy doors closed before sunset when the shrews’ nocturnal feeding habits begin. The researchers have kept quiet about their work, hoping to complete their study before outside forces interfere, convinced that the shrews will eventually cannibalize each other once other prey is exhausted.
Tragedy strikes when [Rook Griswold] is ashore and is killed by the advancing pack of shrews. The storm intensifies, and rain and wind pound the island as the shrews push through a barn floor and begin feasting on livestock. Thorne warns that the walls around the main building are too solid for digging, but the adobe walls of the barn are vulnerable, and the storm’s moisture could turn promising shelter into a trap. The group resolves to make a break for the coast at first light.
A shrew finds a way into the basement through a broken window. Mario and Thorne pursue the creature, and Mario shoots it, but not before it bites him. Thorne treats the wound, but Mario dies within moments. An examination of the dead shrew reveals a frightening detail: its venom is highly toxic, a side effect of the researchers’ use of poisoned bait meant to curb the population.
With dawn approaching and the storm receding, Thorne and Jerry scout a path to the shore. Jerry becomes increasingly agitated and threatens Thorne, ordering him to stay away from Ann. Thorne disarms him, and they radio for the ship to come ashore. When Rook does not answer, the men follow traces of his clothing to the beach to retrieve the vessel. The shrews, starved by the lack of other food, abandon their nocturnal habits and launch a fierce assault on the compound’s defenses. Thorne and Jerry race back to the main building, but Jerry reaches it first and tries to lock Thorne outside. Thorne climbs the fence instead, and the two men clash in a fit of anger that ends with Thorne ramming the gate open and nearly hurling Jerry to the teeth of the creatures.
As the danger intensifies, another shrew bites Radford, and the battle grows more desperate. Thorne quickly devises a plan: lash together several empty 50-gallon chemical drums to create improvised armor, and then duckwalk to the beach to reach the boat. Jerry, hampered by claustrophobia, refuses to crawl inside the makeshift armor, choosing to watch from the roof instead. He later breaks for shore, but the shrews pursue and kill him before he can escape.
Thorne, Ann, and Marlowe reach the shoreline, shed their improvised armor, and swim to the boat. They fight their way aboard and, with the island fading behind them, begin the long journey back to the mainland. Safe at last on the deck, Thorne and Ann share a quiet, hopeful kiss, signaling a fragile turn toward safety after a harrowing ordeal on the cyclone-ravaged island. The sea bears them away from danger, but the price of survival is clear: ambition without caution can unleash creatures beyond control, and loyalty to one another becomes the last shore worth defending.
