After enjoying a picnic with his long-suffering class mates, Bunter, the outrageous Fat Owl of the Remove, insists they take a short cut back to Greyfriars School through an estate, which is strictly out of bounds. Bunter gets split up from the rest of the boys and fails to appear for 'call-over' that evening. The scene is set for a sparkling series of twists and turns involving land-owner Sir Hilton Popper, a pimple faced thief Slim Judson and Bunter's irascible form master Mr Quelch. They, plus the local police and the entire school, become involved in the hilarious mystery of Sir Hilton's missing postage stamp, the precious Blue Mauritius. There is even a rare but welcome appearance of Bunter's sister, Bessie.