When Guy Ritchie’s witty, enjoyable reboot of Sir Arthur Conan Doyle’s iconic detective stories busted blocks back in 2009, a follow-up was unavoidable. Cynics would argue that a visit from that scourge of movie sequels, the law of diminishing returns, was equally inevitable.

Here we find Holmes (Robert Downey Jr) already hot on the trail of his latest nemesis, ‘Napoleon of crime’, James Moriarty (Jared Harris). When the mad professor schemes to have Holmes’s on-off squeeze murdered, our hero spirals into depression – until the return of his trusty sidekick Watson (Jude Law) shakes him out of his torpor and sets him back on the warpath, following a trail of destruction that will lead to Paris, Germany and – inevitably – Switzerland’s Reichenbach Falls.

The best comparison to draw here is with the Pirates of the Caribbean sequels: the cast and crew remain unchanged, but a key ingredient is lacking. Perhaps it’s a sense of spontaneity: where the first film seemed genuinely sprightly and off-the-cuff, the outcome of every thunderous, whizz-bang, CG-fuelled action scene in the sequel feels – that word again – inevitable.

Salvation arrives in the form of Stephen Fry as Holmes’s brother, Mycroft, and while his schoolmasterly demeanour and encyclopaedic intellect isn’t exactly a stretch for Britain’s favourite quizmaster, it’s a welcome distraction from the increasingly stale banter of the two leads. The result is a fitfully amusing but largely unsurprising and uninvolving action-movie-by-numbers: elementary, and not in a good way.