The series follows two children, the dim-witted but kind-hearted Billy and the cynical, cold-hearted Mandy, who win a limbo contest against the Grim Reaper after he attempts to claim the soul of 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 to serve their whims and accompany them on various bizarre adventures. Throughout the series, the trio navigates a world filled with supernatural entities, mythological creatures, and cosmic horrors. While Billy often causes chaos through his sheer ignorance and Mandy exerts her iron-willed dominance over everyone around her, Grim frequently finds himself caught in the middle, struggling to maintain his dignity while being dragged into the children's surreal and often dangerous escapades.