After some children playing in the street break a window of Louie's sweetshop with their baseball, Slip and Sach decide to rent a nearby vacant lot in which the children can play. The property is owned by the Gravesends, who invite the Boys to their mansion to discuss the arrangement. Unknown to Slip and Sach, the family - - two brothers, Derek and Anton, sister Amelia and niece Francine - - are all quite mad. Derek requires a human brain to transplant into his gorilla, Cosmos, while Anton needs a head for his robot Gorog. Amelia seeks food for her man-eating tree and Francine, a vampire, is always on the lookout for new blood. All the Gravesends consider that Slip and Sach qualify as donors for their particular experiments, prompting Derek to invite them to stay the night, stating that he will discuss the rental of the lot with his lawyer in the morning. After Grissom, the butler, is temporarily transformed into a monster when he drinks a potion Anton has created, Derek prepares Cosmos to receive Sach's brain. While demonstrating to Derek that he can control Gorog by sending the robot directions over a microphone, Anton accidentally sends the robot to kill Sach in the guest bedroom. Luckily for Sach, Anton rescinds the order before Gorog can do any damage. Meanwhile, Louie, becoming concerned that Slip and Sach have not returned, sets off with Chuck and Butch to find them. In the morning, at gunpoint, Derek informs Slip and Sach that due to the operations they are about to undergo, they will be the forerunners of a new, happy race, unburdened by intelligence. Alerted that Louie and the others are at the front door, Derek locks Slip and Sach in a closet, from which they attempt to escape by breaking through the wall, but discover that Cosmos' cage is on the other side. After Derek takes Louie and the others prisoner, he and Anton prepare Slip and Sach for their respective operations. Cosmos gets loose, however, and knocks out Grissom and Anton, then frees Slip who inadvertently activates Gorog, telling it to attack Cosmos. Slip then tries to escape, but is cornered by Amelia who wants to feed him to her tree. Slip is then freed by Francine so that he can become a blood donor. Sach escapes only to be chased by both Cosmos and Gorog and is eventually caught by Grissom, who takes him to the laboratory where Derek and Anton fight over who will use Sach in their transplant. Meanwhile, Slip has found Louie and the others and gains control of Gorog, sending it to attack Grissom and the brothers. After Anton destroys the robot's controlling mechanism, they recapture Sach, who drinks some of Anton's potion, turns into a monster and defeats them all. The effects of the potion wear off just as Sach is about to strangle Slip. Slip then secures Derek's signature for the use of the lot. However, back in the Bowery, Louie's windows continue to be broken, thanks to the addition to the team of a powerful home-run hitter, Gorog.
