Taglines: It's loose... It's angry... And it's getting hungry!
Here are the monster's little toys. Once they were little girls and boys.
It is Labor Day weekend, 1946. Young, virginal Ida Parsons innocuously plays with her German Shepherds as her father hosts a raucous party inside their small island cabin. Amid the festivities, an older, drunken man named Tom Rice staggers outside and propositions Ida. When she refuses, he chases her into the woods and brutally rapes her; her dogs, hearing her muffled cries for help, break out of their pen and track her down in the woods, where they attack and fatally maul Ida's rapist. As Tom lies dying, Ida beats him to death with a log.
The credits, accompanied by somber piano music and the distant barking of dogs, roll over washed out, grainy photos depicting Ida's happy childhood. The photos track Ida from an eight-year-old girl up through her early teens, showing tender memories of her youth, from imaginary parties to group shots with her girlfriends. The final, ominous photo is the only one not to depict Ida smiling; apparently taken post-rape, it depicts her stone faced and covered in scars, staring dead-eyed into the camera.
After the credits, the movie picks up in 1982. Preppy brothers Eric and Nick are borrowing their father's yacht to take their girlfriends, Sandy and Donna, on a weekend outing along with their sister, Carla. At the outset of the trip, Nick demonstrates his intent to be the "alpha male" of the trip, insisting that he be the one to pilot the yacht and at one point pulling a gun on Eric to demonstrate his authority. As the tensions rise between Nick and Eric, Donna and Carla engage in girl-talk, and geeky Carla silently laments that she is the sole member of the cruise who came along without a significant other.
That night, fog settles in; Eric and Nick, hearing cries out on the water, discover and rescue a shipwrecked fisherman named Bert. Bert informs them that he wrecked offshore Dog Island, the home of lumber baroness Ida Parsons, who has used her family fortune to hole herself up on the island for the past thirty-five years; now she only makes two annual voyages onto the mainland for necessary supplies, and has never spoken to anyone during these trips. Recovering from the onset of hypothermia, Bert tells the quintet a campfire story about the savagery of the wild dogs which roam Ida's island, acting as her sentries. The story, coupled with the cries of wild animals coming from the nearby island, startle Nick enough that he runs on top of the yacht and tries to speed back to the mainland; instead, he wrecks it, damaging the fuel line and causing it to explode.
Donna, Eric, Sandy, Nick, and Bert wash up on Dog Island; Bert has been mortally wounded, and Carla is nowhere to be seen. Seeking to reclaim his macho status, Nick wanders off into the woods, and is subsequently beaten to death by a hulking figure. The next morning, Sandy and Eric go off onto the island, hoping that Ida Parsons will help them get back to the mainland. Shortly after they leave the beach, Bert goes into shock, and Donna desperately tries to warm him by stripping to the waist and lying topless across his shirtless torso; seconds later, the same figure which killed Nick charges Donna and Bert, fatally hurling Donna against a rock wall and disemboweling Bert.
At the center of the island, Sandy and Eric discover Ida's fortified cabin, as well as the fact that all of Ida's dogs have died long ago, their mutilated skeletons lying strewn across the compound lawn. In Ida's boathouse, the duo discover Carla, who washed up at another point on the beach and made it to the compound in the middle of the night. Sandy and Eric direct her to Donna and Bert, but en route to the camp she's murdered by the same individual who killed Bert, Donna, and Nick.
In the course of exploring Ida's compound, Eric and Sandy discover a dust-covered nursery full of antique toys, and a cobweb-covered crib; they also discover Ida Parson's diary, which contains insane, rambling passages about giving birth to a "pure" child, which she intends to keep sinless by secluding him from all the evils of the outside world. Eric and Sandy figure something traumatic must've happened to Ida to cause her to write such things, and reach the conclusion that Ida's son has died, causing her to become delusional. As they continue exploring the house, Sandy comes across Ida's skeletonal corpse, positioned in repose in her bedroom. The pair become frightened, and decide to collect supplies and head back to the shore to collect the rest of their party so they can formulate an escape from the island in Ida's old rowboat. When night falls, however, the pair are attacked by the hulking figure, and barely escape by hiding in Ida's house.
Eric and Sandy decide that it must be Ida Parson's son, left insane by his life of solitude under the care of the imbalanced Ida; with nothing to do but learn from Ida and explore the wilderness, he's become immensely strong, a capable tracker and hunter, and is thoroughly convinced that all outsiders are a threat to him and his mother. With the death of Ida, he was left without any basis for reality, and ended up eating the dogs to survive-- and has now turned to people.
Eric and Sandy go back to the house and get the matches that Sandy dropped earlier. Ida's son breaks down the door and Eric manages to hit the mutant with the branch. Eric thinks he got him but the mutant is still alive and hits Eric and picks Eric up, wraps his arms around Eric's back and crushes him, breaking Eric's back. Eric is killed. Sandy seen the mutant laying Eric's corpse on the ground. Sandy goes up to Ida's bedroom and wraps a blanket aound her head, playing Ida. The mutant leaves, but when Sandy leaves the room, the mutant is there and attacks Sandy, she runs from the house and to the boathouse. Sandy runs into Carla at the boathouse and the mutant grabs Carla and crushes her face and kills her. Sandy manages to lure the man into Ida's boathouse, which she sets on fire; Ida's son is mortally burned in the blaze, but still manages to attack Sandy, who finally kills him with a signpost. When the mutant tries to attack her, he gets stabbed with the signpost and is now finally dead.
Traumatized by the death of her friends and the murder she has been forced to commit, Sandy sits alone on Ida's dock -- a recreation of the opening credits shot of a scarred and traumatized Ida.