When Billy's dad gets a promotion at his job, he tells his family they are going to move to another town, starting a dispute between Billy and Mandy over who gets custody of Grim. Grim suggests they settle this in court, and hours later Billy, Mandy and Grim find themselves in the Underworld court, presided by Judge Roy Spleen, and which has Fred Fredburger, an incorrigible dumb character, as a juror.