Timid Milton and his older, scofflaw sister Marlo die in a marshmallow bear explosion at the mall and are sent to Heck, an otherworldly reform school.

When a freak marshmallow explosion sends both Milton and Marlo Fauster to Heck, a reform school run by Bea 'Elsa' Bubb, the Principal of Darkness. Although his sister, a naughty kleptomaniac, deserves to be there, Milton believes he shouldn't. Now the siblings must find a way to escape before they are stuck in Heck for eternity.