S30E3 - The Locked Room Mystery

The door had been locked from the inside. The windows were barred, locked from the inside, didn't open and were made of brick. All the books were chained to the shelf save one, open on the table at a page entitled 'How to secure a room from the inside'. All the boxes had been ticked. And even the pen used to make the ticks was one of those ones on a little chain you get in banks.- John Finnemore, Souvenir Programme