Following an end-of-term school ceremony, the American boy Bobby decides to go with his friend Akkun into the mountains outside their village, to a place perfect for a secret base. On the way they stop into a mysterious amusement park. They have fun there, but are attacked and cannot leave. Falling in with some other children who are also lost there, they fend off a number of attacks, and gradually learn the truth behind the facility. It was put there by a man named Koike as a trap for passing children. His son Takashi needs to feed on human children in order to survive.
Akkun is killed, and Bobby is captured with Tokiko, a girl they have met there. She disappears, and Koike goes to work on Bobby, trying to fatten him up. Bobby manages to escape. With the help of Saruyoshi, a mysterious old hermit, and the animals of the forest, Bobby builds up his strength and returns to the facility in an attempt to save Tokiko.