On <em>Labor Day weekend</em> in 1946, a young woman named Ida Parsons is chased into the woods and raped by a houseguest. Her dogs break out of their pen and attack and kill Ida's rapist, a brutal event that casts a long shadow over the island’s history.
Fast forward to 1982, when brothers Eric Simmons and Nick Simmons borrow their father’s yacht to take their girlfriends, Sandy Ralston and Donna Blake, and their sister Carla Simmons on a weekend outing to St. Martin Island. That same night, [Eric Simmons] and [Nick Simmons] rescue a shipwrecked fisherman named Bert Defoe, who was wrecked offshore Dog Island—the secluded home of lumber baroness Ida Parsons, a woman who has holed herself up there for thirty-five years, rarely venturing to the mainland and never speaking to anyone. Bert, recovering from hypothermia, shares a campfire tale about the wild dogs that roam Ida’s island as if they were sentries.
The group’s boat crashes during the voyage, and they and Bert wash ashore on Dog Island. Bert is wounded, and Carla Simmons remains missing. Nick Simmons wanders into the woods and is killed by a hulking figure, a portent of the danger lurking on the island. By the next morning, Sandy Ralston and Eric Simmons venture onto the island hoping Ida Parsons will help them return to the mainland. As they move through the landscape, Donna Blake tends to Bert’s injuries while the rest of the group searches for a way off the island.
The same unseen figure that killed Nick then attacks Donna and Bert, decapitating Bert in a brutal moment that underscores the threat surrounding them. In the center of the island, Sandy and Eric discover Ida’s fortified cabin, a stark symbol of the isolation that has shaped the place for decades. Ida’s dogs lie dead in their pens, and in Ida’s boathouse they uncover Carla hiding under a tarp, having washed up at another point and making it to the compound during the night. Exploring Ida’s compound reveals a dust-covered nursery filled with antique toys, a crib, and Ida’s diary, whose rambling pages speak of birthing a sick child and keeping him "sinless" by secluding him from the outside world.
Sandy eventually encounters Ida’s skeletal corpse in Ida’s bedroom, and the group resolves to escape the island using Ida’s old rowboat. As they prepare to flee, they discover the bodies of Nick and Donna in the basement and flee to the beach in a panic. Eric and Sandy piece together Ida’s history: the 35-year isolation has produced a mutant son who has grown up to be a powerful hunter, convinced that outsiders are a threat to him and his mother. With Ida dead, the son is left without any tether to reality, and he begins to turn the world inside out to survive—eating the dogs to sustain himself after years of solitude.
Back at the house, the group recovers supplies and Sandy retrieves matches she dropped earlier. The mutant attacks once more, forcing Eric to defend himself with a broken branch, but the creature overpowers him and fatally breaks his back. The chase leads Sandy upstairs to Ida’s bedroom, where she, channeling Ida, manages to coax the hunter to leave. As Sandy escapes, the mutant pursues her to the boathouse, where she unexpectedly runs into Carla. The creature seizes Carla and destroys her, prompting Sandy to set Ida’s boathouse alight. Severely burned, the mutant still closes in on Sandy, who finally impales him with a sharpened signpost.
The ordeal leaves Sandy scarred by the violence she has witnessed and the brutality she has inflicted. She sits alone on Ida’s dock, a stark image that mirrors Ida’s own lifetime of isolation and trauma, and the island’s legend of wild guardians becomes a haunting reflection of the survivors’ grim reality.
