It’s 1947 and 93-year-old Sherlock Holmes (Ian McKellen) grapples with a failing memory as he strives to reconstruct a troubling 35-year-old case.

Arthur Conan Doyle’s iconic detective has inspired a flurry of revisionist reinventions recently, but this tender, melancholy drama about the great man in extreme old age – based on Mitch Cullin’s 2005 novel A Slight Trick of the Mind – might just be the most surprising.

As Holmes reconstructs the case that precipitated his retirement to the Sussex countryside, there is also another mystery to solve, involving his cherished bees and his widowed housekeeper’s bright young son (Milo Parker, very impressive).

These puzzles may lack the fiendish elegance of the best Holmes mysteries and their unravelling proceeds at an understandably gentle pace.

Yet Sherlockians will still find much to relish. McKellen is superb, both as the halting nonagenarian and, in flashbacks, as the spry, supremely confident detective of 35 years earlier.

In a nice, teasingly self-referential touch, McKellen’s Holmes drops in on a matinee screening of a black-and-white 1940s Sherlock Holmes adventure whose camped-up, silver screen detective is none other than Nicholas Rowe, star of 1985’s juvenile romp, Young Sherlock Holmes.