St Saviours schoolgirl, May Lewis, is found with a serious head injury in the boiler room of the adjoining boys school. Father Brown discovers the weapon, a cricket bat, and Inspector Mallory arrests the owner; schoolboy Daniel Gates her boyfriend. In hospital May is attacked again and her suspected attacker, gardener Jack Coll, is found dead. Father Brown uncovers years of bullying at the school and the use of a modified cat o' nine tails.