Here's something you won't often find at the Fringe: a period mystery thriller, played with a perfectly straight bat.  No sly subversions, no fourth-wall shenanigans, no heavy-handed modern parallels; just a well-conceived and satisfying riddle, starting with a body on a railway track.  Set shortly after the Second World War, Kieran O'Rourke's script studies a society grappling with hidden truths, yet he never allows his subtly-handled themes to get in the way of an engaging storyline.

The eponymous Hector Cartwright is the only real suspect here; a wise decision, given the play's relatively short running time.  At first, the mystery lies in whether this upstanding figure can really have dunnit, and what the "many crimes" mentioned in the title could conceivably be.  In due course, however, another possibility unfolds – has our man been framed? – and the ultimate resolution is pleasingly unpredictable, as contrasting echoes of Cartwright's past add up to genuine frissons of doubt.

The characters all fit comfortable patterns, yet are drawn with enough detail to make you care.  Much of the interplay is between the three police investigators: one is a set-in-his-ways rural inspector, one is a go-getter from London, and the last, less conventionally, is a woman constable, brought in under equality legislation introduced after the War.  It's a well-worn set-up, but O'Rourke gives it a gentle twist, with a past favour casting a subtle shadow over the interrogation to come.

Alongside this resolutely traditional detective story, we witness scenes from a farmhouse in wartime France, where a pair of sisters quarrel over whether to resist or collaborate with the occupying German army.  The quirky scene changes – which see characters from both eras briefly mix on stage – lend visual interest to what could easily have been a very static play, and help tie the two settings together.  But I fear that O'Rourke leaves it a little too long to explain how these seemingly disparate storylines are connected; it's only at the moment of revelation that his story grows truly intriguing.

The script's also weighed down a little by the sheer number of facts it needs to convey, ranging from Second World War history to the mechanics of operating a railway line.  The pace does dip noticeably from time to time, so a trim of some of the nerdier detail might help keep things moving along.  And the twist ending's unlocked by a clue we've never seen before, a gambit which aficionados of detective fiction rank as highly unfair; but it's guessable anyway and I thoroughly failed to guess it, so I'll give O'Rourke a grudging pass on that one.

All in all, The Many Crimes Of Hector Cartwright is a finely balanced production.  As a mystery, it has just the right amount of intricacy, with carefully-planted seeds of confusion satisfyingly resolved by its conclusion.  But it touches on deeper topics too, especially where it highlights the harsh reality hidden under tales of wartime derring-do.  There's room for some sharpening, and a little more tightness to the performance – but this is already a classic treat for mystery-lovers to enjoy.