When timid Milton and his older, scofflaw sister Marlo die in a marshmallow bear explosion at Grizzly Mall, they are sent to Heck, an otherworldly reform school from which they are determined to escape.

Eleven-year-old Milton Fauster puts aside his own escape plans to help his sister, Marlo, who is in training to be an underworld secretary, and his best friend, Virgil, who is stuck in the circle of Heck reserved for overweight children.

Eleven-year-old Milton and his older sister Marlo are led by the Glib Reaper to Camp Snivel, the level of Heck for whiners, but manage to escape, only to be caught and taken to the court of Judge Judas to testify at Satan's trial.