The Six Thatchers

Six head statues of Margaret Thatcher are being smashed, one by one, and Sherlock Holmes is handed the case. It looks like vandalism, or the work of a madman, and the episode has a good time letting it look that way for as long as it can. The targets are so specific that the randomness is the first clue. The question at the centre is whether a darker purpose sits behind the damage, and the answer, when it comes, is much darker than a political statement.

The investigation takes Holmes through London's underbelly, chasing a pattern the police read as random. The trail pulls in Sir Edwin and Lady Smallwood, and a man called Ajay, and it keeps coming back to the same question: why these six? Each smashed bust is a message, and the message is aimed at someone who doesn't know they're being spoken to. The episode is a slow burn that rewards patience, and the statue angle is a clever piece of misdirection, because it looks like politics and turns out to be something far more private.

Mark Gatiss, Steven Moffat and Arthur Conan Doyle wrote it, Rachel Talalay directed, and the usual company is in place: Benedict Cumberbatch as Sherlock, Martin Freeman as Watson, Una Stubbs as Mrs Hudson, Rupert Graves as DI Lestrade, Mark Gatiss as Mycroft, Louise Brealey as Molly, Amanda Abbington as Mary, Lindsay Duncan as Lady Smallwood, Simon Kunz as Sir Edwin and Sacha Dhawan as Ajay.

It is not the show's most beloved hour, and it doesn't need to be. It is a proper detective story that keeps its secrets until the end, and it gives Mary more to do than anyone expected, which turns out to be the point of the whole season. And when the truth finally surfaces, it lands on the people at Baker Street rather than the public, which is where this show always hurts the most.