As a young boy in Japan, Kenji is left home alone and plays Hitori Kakurenbo, a game of hide-and-seek with a demon, an Oni. His family comes home and the demon kills them all. A traumatized Kenji ends up in the foster system, with no memory of what happened that night. The doctors convince him he witnessed a human murder his family. Several years later he's adopted by a US family. His adoptive father, Rob, also witnessed the demon when he was young, and still has nightmares about El Cuco, the boogeyman of Puerto Rico.
When local kids start going missing or turning up dead, their bodies violently torn apart and half-eaten, Rob begins investigating the cases. His nightmares return, and Ken begins having them, too. After seeing a psychiatrist about their nightmares, Rob's premonitions start coming true and he fears he may be the murderer.
Is Rob the boogeyman or is Ken right and the Oni has returned?