This Can't Be Happening at Macdonald Hall

The two main characters, Bruno Walton and Melvin “Boots” O'Neal are two small-time troublemakers who share a room at the boarding school Macdonald Hall. Across the road is a girl's boarding school, Miss Scrimmage's Finishing School For Young Ladies. Best friends, they play mischievous pranks on the school, faculty and other students. They are constantly under the watch of Headmaster William R. Sturgeon, nicknamed “The Fish” due to his surname but also due to the trademark stern, fishy-like stare he bestows upon his students whenever he disapproves of them.

Following the abduction of an overweight cat mascot of a rival hockey team (in an attempt to demoralize them), Sturgeon comes up with the decision to forbid them from seeing each other and separating them completely. Bruno moves in with the school genius, Elmer Drimsdale, while Boots is placed with wealthy hypochondriac George Wexford-Smyth III. The two both realize they can't stand their new roommates and decide to meet at a cannon at night to discuss ways of getting back into their old room together.

The two prominent ideas they have, including having both Elmer and George complain to the headmaster to get them to move elsewhere, and then framing their roommates to have themselves moved away from them, only get them into more trouble.

Eventually they come up with the idea to study hard to get into the Honor Roll to show they're capable and should be able to move back in together as a reward, but the plan backfires after Sturgeon attributes the boy's resulting high marks as the result of him separating them. In desperation they meet again, but while en route to the cannon, they spot an entangled hot air balloon stuck in a nearby tree and find a boy stuck up there. They rescue the boy (named Francisco) using a volleyball net and take him to Sturgeon, who realizes he's the son of an important Ambassador from Ottawa who got lost in a balloon during the day. They are then interrupted by Elmer Drimsdale, who had witnessed the balloon in his telescope and had concluded it was a UFO. He goes off and causes a massive disturbance between both schools.

The next morning, the Ambassador arrives at the school to retrieve his lost son and goes on to honor the people responsible for rescuing him – Bruno, Boots, and Elmer, who receive medals from the RCMP, the Ontario Provincial Police, and from the Ambassador himself, who is ironically representing the country of Malbonia, the country of which flag Bruno and Boots had used in a prank earlier on in the novel. For Macdonald Hall to honor them, Elmer receives a new telescope and Bruno and Boots get their wish – to share a room again.