At the prestigious private school Dartmoor Academy, the stern principal Morgan Brinway pushes the second-graders to study opera appreciation. Five feisty kids — the bold Sonny, the inventive Loaf, the movie-loving Domino, the compassionate yet tough Lucy, and the reluctant Witz — form the cohort nicknamed the "Stinkers" by Mr. Brinway and secretly skip class to turn the school grounds into a playground of mischief.
Their latest caper begins with a plan to power a flying chair design. The Stinkers raid Groundskeeper Roy's shed for a leaf blower, after earlier swiping Mr. Brinway's desk chair. The stunt goes awry when the leaf blower isn’t fully secured to the chair, and the device erupts into the air, leaving Witz perched in the chair as it zooms toward Mr. Brinway's shiny new convertible. The machine runs out of gas just in time to spare the car, but Roy’s lawn mower’s edging blade still tears a side door. A disappointed-but-daredevil Mr. Brinway warns the Stinkers they will be expelled if they mess up again, and the kids, far from giving up, double down on their mischief while trying to stay out of sight.
A school aquarium field trip sparks a bolder scheme when the group encounters a sea lion named Slappy. Sonny and the others decide to smuggle Slappy back onto the bus and stash him in Mr. Brinway's hot tub as a form of celebration. When Mr. Brinway comes home earlier than expected, the Stinkers retreat Slappy to Witz's house for safekeeping. Roy, mistaking Slappy for a giant gopher, resolves to get rid of the intruder, while an animal broker named Boccoli hatches a plan to steal Slappy and sell him to a Bulgarian circus. Boccoli’s efforts prove clumsy and unsuccessful, fueling the ensuing chaos.
The next day, the Stinkers take Slappy to the beach to release him, but the sea lion resists leaving the water. They notice an orca nearby, which they suspect is Willy from Free Willy, and realize Slappy fears the ocean because orcas prey on sea lions. Sonny proposes returning Slappy to the aquarium, but a Dartmoor festival intervenes, and Roy’s attempt to dispose of Slappy disrupts the festivities. Mr. Brinway, convinced the Stinkers are behind the trouble, expels them.
The truth emerges: Boccoli has kidnapped Slappy. The Stinkers rally to rescue him, tracking Boccoli to his hideout and turning the tide with a trio of comic yet clever moves — spraying water, coating his body with sawdust, and firing Roy's gopher bomb gun. After freeing Slappy, the group is forced to flee Boccoli’s pursuit. A high-stakes chase ends with the Stinkers and Mr. Brinway crashing down a log flume; Boccoli is knocked off course and ends up stranded on a floating log, while Roy ties a rope around him to slow the escape. Slappy is returned to the aquarium, and Mr. Brinway withdraws the Stinkers’ expulsion, sealing their status as unlikely heroes who have learned to balance mischief with responsibility, teamwork, and a newfound respect for the consequences of their actions.
