After his last misdeed, rebellious 11 years old Stephan is sent by his busy flight attendant mother to a boarding school, Schreckenstein Castle, directed by her friend Gerhard "Rex" Maier.

However, he gets eventually accepted into their group, which gathers in the castle's secret torture chamber, and is subjected to a test in order to prove he is worthy, namely aiding them to pull a practical joke on a rival board school, Rosenfels.

The girls pretend to make peace with them in order to steal their key of Schreckenstein, albeit unexpectedly having fun all together during the truce, and later infiltrate the school to thrash the boys' clothes.

Humiliated by the twist, and being secretly motivated by Rex, the boys pull another prank on Rosenfels, this time sneaking in a drone, piloted by Stephan due to his experience with them, which activates the castle's fire alarm system.

The damage caused by the water hydrants, however, reveals that Rosenfels' beams are rotten, which drives Rex and Mrs. Horn to move the girls to Schreckenstein while the building is repaired.