What is happening to Sherlock Holmes and Dr Watson? They have been updated on television, by Benedict Cumberbatch and Martin Freeman, having endured the mixed blessing of a louche makeover by Robert Downey Jr and Jude Law in last year's movie.
Anyone dissatisfied with these treatments will probably be relieved by the news that Jeremy Paul's clever two-hander restores not only the language of Arthur Conan Doyle, in the right period, but also the civilised decorum of male friendship. The only major liberty is an outlandish twist in the story of Professor Moriarty, the Napoleon of crime.
Film fans still think of Basil Rathbone or Peter Cushing as the great detective. Robert Stephens (with Colin Blakely as Watson) attracted a cult following in Billy Wilder's film. The best I have seen was John Wood in the RSC's long-ago revival of The Private Life of Sherlock Holmes.
Peter Egan and Robert Daws are surprisingly good, though, in this sudden summer filler at the Duchess. They are both fine actors: Egan has a monumental quality and he finds a troubled, furtive oddness in Holmes's sudden pauses and confessions. Daws's willing Watson, tweedy and ruddy-faced, vocally dapper and sharp in his twitching, is almost the perfect foil.
Once the audience has conceded that this is a play about a relationship, not a detective story, they are gripped. For quite a lot happens at 221B Baker Street, as realised in Simon Higlett's marvellously cluttered design of spiral staircases, bookshelves, curtains and bric-a-brac, lit atmospherically by Matthew Eagland.
The play was first produced in 1988 and was seen as an attempt to cash in on the television series then showing, with Jeremy Brett and Edward Hardwicke, who took the stage roles too. It was soundly trounced. This is not a humdinger by any means but it has an insistent, engaging quality of humour and truthfulness and a lot of poignancy too.
The twists and turns are well handled by the director, Robin Herford, aided by some tremendous music by Matthew Bugg. Can this be what really happened at the Reichenbach Falls and afterwards? Not so much elementary, as elemental.
To 11 September (0844 412 4659; duchesstheatre.co.uk)