The Alchemist's Secret (S05E10)

Professor Ambrose goes to see his old friend Father Brown 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 and Bunty accompany them to Gloucester University where Principal Malcolm allows student Timothy to borrow a ceremonial key. When the absent minded Professor goes missing, Father Brown sees if he's with Timothy. (iTunes)