The series follows two children, the dim-witted but optimistic Billy and the cynical, cold-hearted Mandy, who win a limbo contest against the Grim Reaper after he attempts to claim Billy's pet hamster. As a result of his defeat, Grim is forced to become their best friend for eternity, bound by a supernatural contract. Living in the suburban town of Endsville, the trio finds themselves in bizarre, supernatural situations that often involve mythological creatures, monsters, and the afterlife. While Mandy uses Grim's powers to manipulate the world around her, Billy's curiosity and stupidity frequently cause chaos, forcing the reluctant Grim to constantly clean up their messes while navigating his own existential dread.