The Sign of Three

The biggest challenge Sherlock Holmes has ever faced is a best man's speech. That's the joke, and the episode has the sense to play it straight: John Watson is getting married, and Holmes has to stand up in front of the guests and say something human about the man he shares a flat with. For a detective who has faced bombers and masterminds, it is, in its own way, the bravest thing he does. The speech becomes a story, the story becomes a review of old cases, and the review surfaces an unexpected mystery.

The wedding gives the show room to do what it does best: fold a proper case into the middle of ordinary life. The wedding is the frame, and the mystery that emerges from the old cases is the engine. The episode is built on a simple gamble, that the audience cares enough about John to sit through his wedding and enough about Sherlock to watch him flail at being normal. Both halves pay off. There's a genuine warmth here, a sense that the show is celebrating something rather than setting a trap, and it lands all the better for it.

Steve Thompson, Steven Moffat and Mark Gatiss wrote it, Colm McCarthy directed. Benedict Cumberbatch and Martin Freeman lead, with Una Stubbs's Mrs Hudson, Rupert Graves's DI Lestrade, Mark Gatiss's Mycroft, Louise Brealey's Molly, Amanda Abbington's Mary, Vinette Robinson's Sgt Sally Donovan, Lara Pulver's Irene Adler and Alistair Petrie's James Sholto around them.

Holmes's speech is the whole episode in miniature, and it works the same way: it starts awkward and ends somewhere true. It is the most romantic hour the show has done, and it gets away with it because the sentiment is earned. John deserves a good day, and the episode gives him one. And Sherlock, for once, is happy to be the supporting act.