Spiv "Flash Harry" has set up a marriage agency for the Sixth Form and an Italian Prince is interested in meeting the girls. Meanwhile, the father of one of the Sixth Formers is involved in a diamond theft and recklessly decides to hide out at the school.
Cheating in an academic competition so that they can win a 'goodwill' trip overseas to pave the way for the Sixth Form girls to meet the Prince, the girls force the diamond thief to reluctantly masquerade as their headmistress so that he can accompany them on their trip as their 'chaperone'.
Chaos reigns as the St. Trinian's girls run amok in Europe, leaving a string of victims in their wake as they win by underhanded means at hockey in France and at water polo in Italy.