HUMONGOUS

During Labor Day weekend in 1946, young, virginal Ida Parsons innocuously plays as her father hosts a raucous party at his home on Lake Michigan. 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 break out of their pen and they attack and fatally maul Ida's rapist.

Thirty-six years later in 1982, Waspy 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 to St. Martin Island. At the outset of the trip. As the tensions rise between Nick and Eric, Donna and Sandy 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; only making two annual voyages onto the mainland for necessary supplies, and never speaking 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. Wrecking their boat after Nick, in a panic, attempts to steer the boat back to the mainland.

Donna, Eric, Sandy, Nick, and Bert wash up on so-called "Dog Island;" Bert has been seriously wounded, and Carla is nowhere to be found. Nick wanders off into the woods, and is subsequently killed by a hulking figure that breaks down the shed where Nick hides. 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. Seconds later, the same figure which killed Nick sneaks up behind Donna and Bert, fatally hurling Donna against a rock wall and decapitating 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 in their pens. In Ida's boathouse, the duo discover Carla alive hiding under a tarp; she apparently washed up at another point on the beach and made it to the compound in the middle of the night.

In the course of exploring Ida's compound, Eric, Sandy and Carla 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 sick child, which she intends to keep sinless by secluding him from all the evils of the outside world. As they continue exploring the house, Sandy comes across Ida's skeletal corpse in repose in her bedroom. The group decides 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. While exploring the basement, they discover the corpses of Nick and Donna and flee to the beach in a panic.

Eric and Sandy deduce that Ida Parson's 35-year-old mutant son was the one behind the murders, 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.

Eric and Sandy go back to the house and get the matches that Sandy dropped earlier. Ida's son attacks, breaking down the door. Eric attempts to fight using a broken branch, but the mutant grapples with him and fatally breaks his back. He then chases Sandy upstairs into his mother's bedroom, where she wraps a blanket around her head and, playing Ida, convinces the mutant to leave his mother's bedroom. When Sandy leaves, the mutant pursues her to the boathouse, where she runs headlong into Carla. The mutant grabs Carla and fatally crushes her face. Sandy manages to lure the man into Ida's boathouse, which she sets on fire; although severely burned in the blaze, he still manages to attack Sandy, chasing her up a hill where she yanks a sharp signpost from the ground and impales him. As he dies, his burned, deformed face is finally seen.

Traumatized by the deaths of her friends and the killing she has been forced to commit, Sandy sits alone on Ida's dock, strongly resembling a scarred and traumatized Ida.