THE sleuthing is far from elementary in Guy Ritchie’s action-packed sequel to his 2009 reinvention of Sir Arthur Conan Doyle’s detective.

The dense narrative taxes what Hercule Poirot referred to as the “little grey cells”, connecting minuscule clues in Sherlock Holmes’s mind through high-speed flashbacks in a style that will be familiar to fans of the TV series CSI: Crime Scene Investigation.

But sinewy plot strands will probably tie younger audiences in knots.

For all its intellectual rigour, A Game Of Shadows often feels like a series of ambitious action set-pieces which have been bolted together by screenwriters Michele and Kieran Mulroney.

Explosions and stunts are orchestrated at breakneck speed by Ritchie, whose repeated use of slow-motion bloats the film’s cumbersome running time.

Holmes (Downey Jr) has cut a swathe through the criminal fraternity of late 19th century London aided by trusty sidekick Dr John Watson (Law), who is bidding farewell to crime-solving to marry Mary Morstan (Kelly Reilly).

Newspapers are filled with shocking headlines about anarchist bombings in Strasbourg and Vienna, the apparent overdose of a Chinese opium dealer and the death of an American steel magnate.

Holmes deduces these events are linked to Professor James Moriarty (Harris) and the sleuth persuades Watson to join him on one final globe-trotting adventure.

En route, the double-act encounters Gypsy fortune teller Sim ( Rapace) and Holmes’s brother Mycroft (Fry), who is well-connected in the British government.

A bruising battle of wits with Moriarty becomes personal when the diabolical professor promises to make Watson and his new wife suffer for Holmes’s meddling.

Sherlock Holmes: A Game Of Shadows is a sporadically entertaining jaunt.

Downey Jr and Law ease back into familiar roles as the quixotic genius and his strait-laced foil, Rapace is suitably feisty in her first English-speaking role and Harris chews on every syllable with menacing intent.

Eddie Marsan and Geraldine James reprise their roles as Inspector Lestrade and long-suffering landlady Mrs Hudson respectively, but neither is afforded sufficient screen time to make an impact.

Fry is a delight, too, as pompous, pontificating Mycroft, including one hysterical sequence which sees Holmes’s sibling wander naked about his home, sparing his blushes (and ours) with strategically placed furnishings.

Thank heavens for small vases.