Dennis is a 30-year-old man with a disability who has had nightmares all his life, dating back to childhood. One recurring dream centers on a carnival ride that seems ordinary, until he is told that
> the ride isn't over.
As an adult, he experiences another nightmare in which a living demon, called the Cadaver, fastens a zipper onto his back and seems to invade his body. He discovers a hidden room in the cellar containing a mirror that he believes is connected to the Cadaver's origin. He begins speaking to his reflection, and the reflection seems to reply back.
John, his brother, also carries his own burdens. John's girlfriend, Lydia, pushes for marriage but is frustrated that John won't address Dennis. A social services worker, Mildy Torres, arrives at their home with concerns about Dennis and recommends that he be taken away, which unsettles John since he feels Dennis is his responsibility. Dennis keeps having dreams about the Cadaver, and his reflection suggests that to get rid of it, they need to begin killing animals. He begins working in the secret cellar, and John is perplexed by Dennis's fascination with videos on taxidermy and embalming.
The killings escalate to children, and the reflection grows more capable, revealing that it was the one behind the nightmares and that there is more to come. Dennis encounters his crush, Susan, at his favorite ice cream shop and musters the courage to ask her out. The encounter is awkward, and Susan declines, which hurts him. Later, when he returns, he tries to frighten her, sending her into the street where she is struck by a moving vehicle and killed.
After a night out, John and Lydia realize they cannot recognize anyone, suggesting a wider danger. Torres returns with two officers, and they are all killed by Dennis. John then reveals a painful secret: Dennis was once a child prodigy whose works had appeared in newspapers. During an argument, John punched Dennis, who fell down the stairs, and after awakening his mental state deteriorated, leaving John with guilt.
Lydia goes to fetch Dennis, but Pete, Pete, a friend of John, reveals that the taxidermy and embalming tapes are actually Dennis's, and he calls the police. When Lydia returns, she is stabbed by Dennis, who proceeds to remove her internal organs. John arrives and enters the cellar, where he sits in a chair with a spotlight on him. Dennis appears with a now-stuffed Lydia and performs a ventriloquist act, while John realizes he is glued to the chair. The other victims are displayed as part of a grand diorama, leaving it unclear who is hallucinating or what is real.
Overwhelmed, John shoots himself. Dennis is knocked down by a Cadaver-Spider hybrid, but the scene reveals he is actually being restrained by police officers who drag him away. In the aftermath, Dennis is placed in the padded cell of a psychiatric hospital. The reflection still seems to control the situation, while the real Dennis remains trapped in his subconscious, awaiting the next dream of the carnival ride.
