At St. Trinian’s, chaos grips the place as the students run wild under headmistress Miss Amelia Fritton, Alastair Sim, with the teaching staff gone and the Ministry of Education stepping in to keep order with a little help from the army. In the midst of this upheaval, the school’s dashing business associate Flash Harry, George Cole, hatches a scheme that blends romance, ambition, and mischief: he launches a marriage bureau for the sixth form girls, hoping to secure a match for one of them with Prince Bruno, a plan that promises both prestige and profit.
To ride the wave of attention and guarantee the school’s selection, some of the sixth form students infiltrate the Ministry and swap a file filled with negative inspector feedback for one that paints St. Trinian’s in a glowing light. The ruse seems to work, and soon the Ministry reveals plans to bring in a new headmistress from abroad. On the night she is due to arrive, the group is stunned when one of the sixth formers, Myrna Mangan, reveals that her father, Lionel Jeffries, has turned up needing help. He is being sought by the local police, acting on orders from Superintendent Kemp-Bird, and the situation grows tense as the search tightens.
Harry and Myrna improvise a bold plan: they disguise Joe Mangan as Hackshaw, and the real Dame Maud Hackshaw, Judith Furse, is abducted and concealed within the school. Meanwhile, a forged letter from Hackshaw reaches the Ministry, recommending that the girls be allowed to embark on the UNESCO tour. With few options left, the Ministry agrees and calls for volunteers to chaperone the group and serve as interpreters.
Superintendent Kemp-Bird, who has been frustrated by his failure to locate Mangan—wanted for a jewellery robbery in London—receives word of St. Trinian’s plans and decides to act. He assigns Sergeant Ruby Gates, Joyce Grenfell, to go undercover as an interpreter, while urging the Ministry to secure coaches from Captain Romney Carlton-Ricketts, Terry-Thomas, the only coach company willing to transport the students under such pressure. As the departure nears, Mangan remains in disguise with the girls, still in possession of the stolen jewels, and hides them inside a water polo ball, a ruse he hopes will stay hidden from prying eyes, though a sixth form student witnesses part of the operation.
The tour takes the group through major European cities, with Paris and Vienna delivering their share of chaos and spectacle. In Rome, Guido Lorraine as Prince Bruno becomes a focal point of attention, and Romney Carlton-Ricketts expresses interest in Gates as the trip unfolds, though Gates reveals her true identity and her discovery of Mangan among the students. Tension peaks when the water polo match before Bruno turns into a chaotic scramble; Mangan is forced to reclaim the jewels after losing the ball, and a frantic chase erupts as the pursuing students close in on him at the Coliseum.
Eventually, Prince Bruno’s intentions to marry Myrna cause friction with one of her sixth-form rivals, and St. Trinian’s earns a form of vindication through the reward for Mangan’s capture—one that Miss Fritton decides to keep for herself and the school upon her return. Back in England, Gates parts ways with Romney to be with Kemp-Bird, who has been demoted after his high-profile missteps in trying to arrest Hackshaw, leaving him to reflect on the consequences of the school’s unruly reputation and the tenuous balance between authority and anarchy at St. Trinian’s.
