[This is a review for Sherlock season 3, episode 3. It contains SPOILERS.]

After a rather slow start (and middle) to the season, it’s gratifying to announce that the game is definitely back on in Sherlock‘s season finale. Steven Moffat and Mark Gatiss’ wit and humor is on top form as John and Sherlock work to pry more than one client from under the boot heel of blackmail, but what really holds “His Last Vow” together is the introduction of a new villain: Charles Augustus Magnussen.

Lars Mikkelsen puts in a chilling, twisted and brilliantly repulsive turn as Magnussen: media mogul, the “Napoleon of blackmail” and the only man that Sherlock has ever truly hated. Magnussen has built a fortune and a terrifying amount of power through his talent for locating people’s pressure points, and as “The Last Vow” begins we find Sherlock deliberately putting himself in Magnussen’s firing line in order to get close to him.

That’s a polite way of saying that John, after a short separation from his best friend, finds him crashed out in a smack den and looking a little worse for wear. Sherlock’s drug addiction has been alluded to in previous seasons, and it’s both surprising and a little disturbing to witness him relapse deliberately for the sake of a case. There’s no time for twelve-step meetings, however, as an attempt to break into Magnussen’s office and steal blackmail documents brings Sherlock face to face with an unpleasant truth: Mary Watson, née Morstan, is actually an ex-assassin who has been lying to John since the day she met him.

At first this felt distinctly like an “out” for John’s marriage and the challenges it might have presented to his crime-fighting hobby, but instead of removing Mary altogether it actually proves to be a rather brilliant way of reconciling the two aspects of John’s life. Having John take Mary back after finding out that she shot his best friend is a very tough pill for viewers to swallow, but between the writing and the performances by Martin Freeman and Amanda Abbington this delicate issue is handled convincingly.

It’s also difficult to stay angry at Mary when Charles Augustus Magnussen is hanging around and making everyone else in the world seem nicer by comparison. Mikkelsen’s performance is truly enthralling, both understated and shockingly nasty, and Magnussen feels in every way like a worthy foil for Sherlock: a stark embodiment of what similar gifts might have been used for in different hands.

In fact, the only thing that’s disappointing about Magnussen is his death, which felt a little anticlimactic and raises the question of why none of the blackmailer’s bodyguards bothered to check if either of his visitors was carrying a gun. Hiring an amateur security detail is false economy, kids.

Although introduction of such a dastardly villain and a high-stakes storyline might leave less time for some of the funny character moments that the writers have indulged in during the past two episodes, ‘His Last Vow’ actually reveals a great deal more about its central players than a trip to the pub or a game of Operation might have done. Of course, Magnussen’s Terminator-esque HUD that displays the bald details about everyone he meets plays into that, but what’s most interesting of all is finding out what everyone’s pressure point is, and how they react to having it flicked.

“His Last Vow” is undoubtedly the best episode of this season, and some might consider it the best episode of the series so far. Moffat’s penchant for slightly over-the-top melodrama is tempered by a healthy dose of dry wit and the performances of Sherlock‘s excellent cast, and despite some of the shocking events that take place the episode still feels nicely grounded.

As for the ending… well, we needed to have something to talk about while we wait for season 4.

Sherlock season 3 premieres on PBS with ‘The Empty Hearse,’ on Sunday January 19th.