When Casper's been being friendly lately even when playing with a boy named Jimmy, Kibosh: The King of the Underworld has Casper enrolled into a Scare School headed by the two-headed headmaster Alder and Dash. He befriends Ra, a mummy with unraveling issues and Mantha, a zombie girl who keeps falling apart. When Casper discovers the two-headed headmaster's plot to use a petrification potion to turn Kibosh into stone and take over the Underworld and Deedstown, he and his new friends must stop him.