Proud Irishwoman Bridie Quilty journeys to Dublin while World War II rages across Europe. During her travels, she encounters J. Miller, who recruits her as a Nazi spy. She acquires the necessary information that leads to the breakout of a German spy who holds key information about the Allies' newest offensive plans. However, the arrival of British officer David Baynes and his romancing of Bridie lead to unexpected consequences.

The fourth form monsters' latest trick is their best ever – they have burned down St Trinian’s school! As the girls stand trial, the police breathe a sigh of relief, but miraculously the judge's infatuation with a student means the school is freed. For the authorities, it means a new reign of terror as the girls of St Trinian’s regroup with gleeful anticipation.