Stanley Caldwell, a screenwriter living in a cabin at the edge of Summit Lake, engages in a disturbing pursuit that begins with a clandestine discovery. After a phone call with his agent Regan, a hidden room in his house reveals a young woman chained to the floor by her ankle. He greets her and assigns the name Kimberly as he places a plate of food before her, all while she remains unaware that he watches her from behind a mirror.
With unsettling routine, the writer brings her fresh clothes, gags her with an orange, and restrains her hands behind her back. He forcibly whisks her onto his boat, rows to the lake’s center, and ties a cement block around her ankles. Curious, he asks Kimberly how she feels, but the frightened captive cannot answer. He then pushes her under, only to haul her back up by the hair to study her expression before letting her sink again.
Back at the cabin, the ritual continues as Stanley resumes work on his script, drawing on the details of his latest victim. The next morning, he descends to the lake’s depths to tend what he calls his "garden"—a collection of women he has abducted and drowned in a similar manner. His appetite for a new subject leads him to Mallory, a young woman working at a movie theater, whose fear of water intrigues him. He trails her after work, intercepts her with a sudden, violent stop, and swerves into her path, forcing a collision between their vehicles.
Mallory’s escape from him is short-lived: he checks on her and, when she follows, opens the van’s doors to reveal only the sound of barking dogs and a menacing yellow clue that reads, “I’m the person your mother warned you about.” He imprisons the defiant Mallory in the hidden room, a place she refuses to fear, and the game of captivity begins anew.
Stanley tries to bend Mallory to his will with a ruse—promising freedom if she conquers her fear and joins him for a ride on his boat. He blindfolds her and drives in circles to disorient her before they return to the lake. He makes a frantic attempt to drown her, but local police officer Boone, aided by a movie-effects shop couple nearby, intervene and rescue her.
Recovery is slow, but Mallory refuses to stay silent. She suggests a bold plan: create a plaster mold of herself with a camera eye to capture the killer when he returns to his garden. The steely trap works for a moment as Stanley discovers the ruse, but he remains hidden behind a scuba mask as he escapes.
Mallory spends a night in a hotel to recover, only to be found again by Stanley, who promptly readies to drown her anew. Just then, Regan and the film’s director arrive at the cabin, throwing a wrench into Stanley’s carefully constructed scheme. He kills the director with a cleaver and binds Regan with Mallory, marching both toward the lake for another round of drowning.
Boone and a team of divers plunge into the water and manage to free Mallory, while Regan’s body is recovered afterward. Stanley disappears beneath the surface, presumed dead after his leg becomes snagged on a rope in his own garden. Mallory survives the ordeal but remains haunted by the memories of her captor.
In a final twist, the narrative shifts to a different pitch: a movie being presented to a new agent about a killer who buries his victims alive. As the agent presses for how the killer feels, the camera reveals that the speaker is Stanley in a new look, quietly declaring, >I'm still doing the research.
