Professor Ambrose (James Laurenson) goes to see his old friend Father Brown (Mark Williams) to ask for help - he's been followed ever since he came into possession of the diary of the university's architect. It claims that the location of the alchemist's secret is hidden in a riddle - the key to the lock to the door. Mrs McCarthy (Sorcha Cusack) and Bunty (Emer Kenny) accompany them to Gloucester University where Principal Malcolm (Danny Webb) allows student Timothy (Richard Southgate) to borrow a ceremonial key. When the absent minded Professor goes missing, Father Brown sees if he's with Timothy. He's saddened to find Timothy's body in his bedroom, his head bashed in with a polo mallet. When the key that Timothy took turns up in the Professor's room, Inspector Mallory (Jack Deam) arrests him for murder, leaving Father Brown determined to prove his innocence.