Baffle them, frighten them and surprise them — this might be considered the job description for any writer setting to work on a stage thriller. Roger Mortimer-Smith appears to have understood his task well on his first attempt at the genre. Guilty Secret, being given its world premiere at the Mill at Sonning (until July 16), by turns has us desperately trying to fathom what is going on, peeping nervously from behind our hands and asking at its close: “Whoever would have guessed it would turn out like this?” At least that’s what it did for me.

The setting throughout is, the programme tells us, “a lonely farmhouse somewhere in the middle of nowhere” — oo-er, then, even before the action starts. Into this remote property (expertly designed by Dinah England) steps sharp-suited, sharp-minded George (Jeffrey Holland). That he’s sharply spoken, too, soon becomes clear in witty put-down banter with his companion Lennie (Neil Andrew), who is clearly possessed of more brawn than brains.

Since we are talking accomplice rather than companion — these actually being a pair of kidnappers — this is as you might expect. Soon the burly goon carries in, bound and gagged, the shapely form of heiress Charlotte (Katie Beard, pictured right), whom he has chloroformed and bundled into the boot of her car.

Their next move is to telephone her business magnate father to demand a ransom. The motive is pure greed. “I’m not a class warrior out to take revenge on the rich,” George explains silkily. “I just want to become one of them.” The demand — £5m.

How the play proceeds from here I have no intention of revealing, except to say that nothing is ever quite what it seems. This applies, too, to the quartet of characters — a canny detective inspector, “the admirable Crichton” (Philip Childs), having put in an appearance after the interval .

Ably acted under the assured hand of stage stalwart Anthony Valentine, the play makes compelling viewing. There was some raggedness on the opening night, suggesting under-rehearsal, though I feel confident this will now have been eliminated.

For tickets call 0118 969 8000 (www.millatsonning.com).