Last Night's TV: Sherlock, BBC 1

Sherlock and a sex scandal? It's eroticism, my dear Watson (and shockingly good TV)

Where does one start, frankly? With the fact that Moriarty uses "Staying Alive" as a ring tone, during a tense stand-off with his nemesis? With the brisk little flurry of jokes that make up the opening montage, in which Sherlock dismisses a series of clients on the grounds that their mysteries are too dull to command his attention? With his delicious indifference to ordinary social graces? (Two small girls are suspicious because they weren't allowed to see their grandad after his death. "Is that because he's gone to heaven?" they ask. "People don't go to heaven when they die," Sherlock replies impatiently. "They're taken to a special room and burned.") Or, and I promise I'll stop listing things soon, with the fact that that joke turns out to be knitted into the solution of a mystery that genuinely did give Sherlock a hard time?

I think the last pleasure can stand for them all, perhaps. Sherlock is a Rolls-Royce of a popular entertainment, beautifully engineered and beautifully finished. The doors close with a perfect thunk every time and a loose thread is unthinkable.

"A Scandal in Belgravia" resolved the cliffhanger of the last story and then idled deliciously towards the start of a new one, taking its time to refresh our memory of Dr Watson's blog, Mrs Hudson's difficult task as housekeeper (she finds a bag of human thumbs in the fridge's crisper drawer) and Sherlock's savant-like ability to see what is invisible to ordinary men. With another writer you might have got restive at the pace, but Steven Moffat enjoys himself so much in the writing that it's impossible not to join in. Summoned to Buckingham Palace for a briefing on a delicate case, Watson and Sherlock find that it is Mycroft who has called them in: "I'll be mother," says Mark Gatiss as he pours a cup of tea for his younger brother. "And there is a whole childhood in a nutshell," replies Sherlock tartly.

The case involves Irene Adler, a high-class dominatrix whose career providing "recreational scolding" has left her in the possession of compromising photographs of a young royal. Sherlock is given the job of getting the photographs back but finds himself competing for this MacGuffin with mysterious American bad guys and the confusing effects of an unprecedented attraction to Irene herself, a woman who wields a riding crop like a master but is herself more turned on by cerebration.

"Brainy's the new sexy," she whispers huskily to Sherlock, exemplifying the old sexy by entering the room stark naked. She is, it turns out, a follower of the blog and something of a fan, so she and Sherlock are able to flirt over unsolved enigmas. Her nudity is a mind-game, not a desperate bid for attention.

And where does one start, again? With the teasing eroticism of Sherlock and Irene's affair of the mind? With the way that Moffat gets some real poignancy and longing into a story so glossily fantastical? With the fact that barely a minute passes without a line that's worth making a note of? Or the fact that, in addition to lovely performances and great writing, the whole thing is filmed with such invention? What a way to start the year.