[1][2] The film's story focuses on St Trinian's becoming caught up in a train robbery, after the gang who conducted it attempts to reclaim their loot from the building that the students and teachers now inhabit.

[3] Gang leader "Alphonse" Askett, who operates under the guise of a hairdresser, is contacted by his anonymous employer, a secret mastermind, on a plan for a major train robbery.

The gang are instructed to rob a mail van of £2.5 million, and hide the loot at Hamingwell Grange, a deserted country mansion, until it is safe to reclaim it.

Meanwhile, Amber Spottiswood, the headmistress of St Trinian's, has an affair with Sir Horace, the new head of the Ministry of Schools (a fictional government department) who recently took over following a Labour Party election triumph.

Discovering it is part of the proceeds from the train robbery reported in the papers, he decides to claim the reward money from Edward Noakes, an insurance assessor.

Whilst waiting for the school to be preoccupied with a dance routine in the main hall, the gang lose a camera to one of the students, housing a hidden two-way radio.