The Blind Banker

A bank is broken into and nothing is taken, only a symbol scrawled where the loot should be. Then a man is found dead in a locked room, and the same markings turn up on walls around London. The first person to read the cipher dies within hours of reading it, which makes it the rare crime where curiosity is the fatal flaw. The premise is tidy and grim at once: every clue is out in the open, and the danger is in understanding it.

Sherlock and John trace the marks across the city, following a Chinese cipher that keeps leading somewhere new. The trail takes in a good spread of London life, and each conversation adds a piece: Sarah, Molly Hooper at the morgue, Soo Lin Yao, whose knowledge of the code is the closest thing to a key, and Andy Galbraith. The clock keeps running, because every hour spent decoding is an hour closer to the next body.

Mark Gatiss, Steven Moffat and Steve Thompson wrote it, Euros Lyn directed, and the cast is strong around the leads: Benedict Cumberbatch and Martin Freeman, Una Stubbs as Mrs Hudson, Zoe Telford as Sarah, Louise Brealey as Molly, Gemma Chan as Soo Lin Yao, Al Weaver as Andy Galbraith, Bertie Carvel as Seb Wilkes, Daniel Percival as Eddie Van Coon and Paul Chequer as DI Dimmock.

It is the episode where the series proves it can do a code story at a sprint. The locked-room format suits Holmes, the cipher gives the plot a proper engine, and the show has the sense to keep John's life moving in the background rather than freezing him as a witness. And the result is a solid, busy hour of detection that deserves more affection than it usually gets. It lacks fireworks, and that's fine. It's the quiet one, and it earns its keep.