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. Much to his department staff's shock, he willingly provides the school a grant in order to relocate following a fire at their previous building. St Trinian's moves into Hamingwell as a result, which subsequently spook Askett's gang when they attempt to return to recover their loot. Learning of what happened, the gang's mastermind instructs Askett to find a means to retrieve the stolen money without raising suspicions from the school. Askett decides on sending his delinquent daughters into St Trinian's as new pupils, instructing the pair to gather any useful information that the gang can make use of. Unknown to Askett, one of the pupils at St Trinian's comes across some of the stolen money and brings it to Flash Harry, the school's turf accountant. 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. However, Noakes is put off by the secretive manner Harry conducts the meeting under, and decides to keep St Trinian's under observation instead. The gang soon receive instructions to take advantage of an upcoming Parents' Day at the school, and pose as caterers in order to recover the money. 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 pupils, housing a hidden two-way radio. When the camera is brought to Harry in order to be fenced, he and some of the pupils intercept a message for Askett from the mastermind and realise the train robbers are in the school. The gang manage to recover the stolen money and escape, just as the school is alerted to what is happening; while Harry and the pupils chase after the gang, Spottiswood leads the teachers in hopes of claiming the reward money. A chaotic chase with trains soon ensues. While the gang use a stolen train to make their escape, the pupils commandeer a passenger train to pursue them, and subsequently seize a van car from them carrying the stolen money. At the same time, police are alerted by Noakes, and commandeer another passenger train to pursue both. The pupils swiftly manage to evade the robbers and leave them being chased by the police, with the gang cornered at a station. While officers arrest most of the gang, Askett manages to escape in the chaos. Meanwhile, the pupils bring the stolen money to a station further up the railway line, planning to claim it for themselves, but are prevented from doing so by the arrival of more police. However, the officers applaud the girls for recovering it, causing the pupils to be awarded with medals, much to the shock of others that know them too well. ==Cast==