Plot

A mentally ill eight-year-old sees his father commit matricide and has flashbacks years later.

Karl is a young man in his 20s who lives with his sister, Anna, in a large suburban house near Tampa, Florida (we see the Sunshine Skyway bridge at one point). The opening credits depict him walking home alone carrying a fishing rod. As he reaches the house, he enters and starts talking to his mother, who is not depicted onscreen other than as a figure in a wheelchair. Karl sees a cat in the kitchen and panics, apparently terrified of cats, and grows more and more agitated until he picks up a fire poker and beats his 'mother' to death. She is revealed, however, to be a mannequin in a wheelchair. Anna returns home and discovers the mannequin in pieces on the floor and reacts angrily. It seems this is not the first time Karl has done something like this; Anna speaks to him as if he is a child, explaining that their mother has been dead for many years and admonishing Karl for his bizarre behavior. She orders him to remove the mannequin from the house, as apparently Karl has a strange attachment to them. It comes to light that Karl's and Anna's father beat their mother to death with a fire poker as she sat in the wheelchair, and Karl saw the murder happen. After the crime, the father was convicted and hanged. The mannequins aren't the only bizarre aspect of Karl's interests, and we see him masturbating while spying on Anna while she's nude in her bedroom after a shower. He is plagued by nightmares where strange, terrifying figures surround him and speak to him in unintelligible, taunting voices.

Karl goes to a secluded beach and removes his clothes, planning on going swimming, but before he can jump into the water, he is nearly run down by a man on a motorcycle who appears suddenly. This man is Tony, a Vietnam veteran who is traveling the country on his bike. In a strange moment of intimacy, Tony also takes off his clothes and the two of them go swimming together. Tony also teaches Karl how to drive his motorcycle, and Karl is delighted that he has a friend who doesn't treat him like a child. He invites Tony back to the house for dinner, and Anna agrees, although she gives him a somewhat hostile reception. After some verbal sparring, Tony and Anna warm up to one another and there is an obvious attraction between the two of them. This seems to dampen Karl's excitement, as he feels territorial about the both of them.

Tony stays on for several days at Anna's insistence, while Karl's behavior continues to become more and more erratic. Like a mischievous child, he spies on Anna and Tony, and he continues to refer to his mother as if she is still alive. Against Anna's wishes, Karl retrieves their mother's wheelchair from the garage and becomes extremely upset when Anna and Tony attempt to remove it. He also keeps a mannequin secretly stashed in his closet, even though Anna has insisted that he get rid of all of them. Karl has an increasing sense of awareness regarding his family situation as well; since their father was convicted and executed for their mother's murder, the locals in their neighborhood are all aware of what happened, including Karl's perpetual immaturity. Anna seems complicit in this, refusing to force Karl to grow up and become self-reliant.

Karl cunningly gets some outside perspective on himself from the mail carrier, an older man named Mr. Phillips. He invites Mr. Phillips inside for a cool drink, and then begins to question him about why he speaks as if Karl was a child. Mr. Phillips reveals details of his mother's murder that Karl was not previously aware of, suggesting that Anna was an accomplice in hiding the mother's body after the murder. To Karl, this clarifies Anna's reasons for remaining as Karl's guardian and refusing to take him to a professional who may have been able to help him mature. Triggered by this revelation, Karl gives in to his murderous impulses, coaxing Mr. Phillips to sit in his mother's wheelchair and then bludgeoning him to death with a fire poker, the very thing that Karl's father used to murder Karl's mother. Before Anna and Tony can return home, Karl buries the body in the garden behind the house, narrowly managing to avoid detection when they come home.

Unaware that Karl is now dangerously unhinged, Anna and Karl go to bed that night as Karl lies in his bedroom with his mannequin 'girlfriend'. Telling it "I'll be right back," Karl goes to their room and murders them in their sleep. In the morning, he cheerily makes plans to leave the house and strike out on his own, using Tony's motorcycle to escape. The mannequin is with him, the lower half of its body strapped to the bike while the upper half is strapped to Karl's back, in a bizarre appropriation of a human rider facing the rear. As we see footage of him driving away on the bike, a voice over from Karl reads a letter he has left behind for Anna and Tony, as if they are still alive and will find it. The letter explains that he is now free to live his own life, and Karl is clearly happy with this new freedom. However, the bizarre sight of him driving a motorcycle while carrying a mannequin attracts the attention of an officer in a passing police car. A chase begins, ending with Karl escaping and the policeman dead when the car flips over and explodes.

Although he seems to have escaped, Karl drives to a bridge and maneuvers the bike down onto a pedestrian walkway right along the side. Stopping over a deep part of the waterway, he tosses each half of the mannequin into the ocean, slashes his own wrists, and leaps into the water after it. Perhaps attracted by the blood gushing from Karl's body, sharks appear immediately and begin to tear the dying young man apart.