Few theatrical properties have been more lucrative than Agatha Christie's long-running mystery, "The Mousetrap." Consequently, every season or so someone with mayhem and bank notes dancing in his eyes sets about trying to build a better one.

"Corpse!," the London import that opened a four-week pre-Broadway engagement at the Morris Mechanic Theatre on Thursday, isn't it, although you don't want to dismiss it with the back of your hand.

A successful comedy-thriller is, above all, a finely tuned machine. As "Corpse!" now stands (or lies), the comedy is none too sharp and the thrills are less than heart-stopping. But if author Gerald Moon, director John Tillinger and actors Keith Baxter and Milo O'Shea, who carry the bulk of the evening, are willing to devote themselves to some serious tinkering, it is conceivable they could get the whole caboodle functioning properly.

What I want to say is "Call in Tom Stoppard, quick." Not just because "Corpse!" needs high-powered help, but because the script already has a certain sub-Stoppardian flavor. He'd be the obvious man to help out with a plot that wants to mix theater and reality; dialogue that aspires to a kind of literate playfulness; and above all, the protracted intricacies that make "Corpse!" first and foremost a game. He'd also give it authentic wit.

The time is 1936 and the place is a shabby basement flat, inhabited by a third-rate thespian, Evelyn Farrant (Keith Baxter), who, when we first meet him, has just returned from a shoplifting spree in dowager drag. Evelyn's sorry theatrical career ground to a stop after he took to poisoning the actors he was understudying. Now the landlady (Pauline Flanagan) is badgering him for the rent and his only hope for solvency and respectability seems to be killing his arrogant, wealthy twin brother Rupert (Baxter, again).

To that end, he recruits a seedy Irishman (Milo O'Shea), who passes himself off as a major although his nickname, "One-Shot Wally," would indicate a baser lineage. Evelyn will pretend to commit suicide; the major will shoot Rupert ("Aim for the heart, if you can find it," advises Evelyn, with the superiority of the evil Shakespearean monarchs he never played). Then all it will take is a simple switch of bodies: Evelyn can step into the duds of his dead twin and resume life on a grand scale.

That is merely the tip of this particular iceberg. To all the willful confusions of the usual mystery, "Corpse!" adds the confusions engendered by two characters who look exactly alike, so that the befuddled major is not quite sure, when the nasty moment comes, which one he's actually killed. But to accuse "Corpse!" of piling complication upon convolution would be wrong. Why else are we there, if not to have the rug whipped repeatedly from under our feet?

The trouble is that Moon's plot, once it is fully revealed, does not have the ultimate clarity of the truly devious. It may make perfect sense on paper, but on stage it leaves you with too many half-answered questions. Tillinger and his actors are too cavalier with those passing clues we need to unlock the puzzle. Furthermore, there is a hammy overlay to the production that tends to deflate any gathering suspense.

I'm not divulging too much to say that in the second act O'Shea is obliged to dispose of a body, which not only resists his efforts but at one point actually pins him to the bed. The moment is ripe with potential for mordant Hitchcockian humor, but as now staged and played, it registers as a broad vaudeville turn. Unlike "Deathtrap," which may be nobody's idea of a great play but nonetheless astutely hewed a fine line between comedy and terror, "Corpse!" prefers to opt for the laughs. If the actors are not taking their lethal impulses seriously, should we?

Baxter's flashy double assignment is an actor's dream. Expelled from the theater, Evelyn views the world as a stage and his sorry part in it as a starring role. Here, Baxter's overwrought histrionics are just right -- grand, desperate and luminously mad. But when Alan Tagg's set revolves to reveal Rupert's cushy art deco pad, the actor loses dimension, playing the snobby twin as a stiff-backed Noel Coward parody. Before long, the switches are coming with alarming speed. Cleverly orchestrated as they are, they'd be more impressive if Baxter portrayed both sides of this fraternal equation with equal vividness.

O'Shea -- the picture of surprise with those bushy black eyebrows forever arched over that choirboy face -- suggests a kind of bumbling innocence that would be more effective if he didn't cater so obviously to the crowd. Wrongly, I think, he saves all that's dark and treasonable in the character for the final moments. Flanagan, however, has a high old time with the incidental role of the landlady, a slattern in white powder and bottle-red hair, who has a propensity for malapropisms. It would be nice if Moon could entangle her more decisively in the nefarious business swirling about her oblivious head.

Nothing is more calculated than the stage thriller. It is a device that tricks us into looking left when the surprise is coming from the right, and encourages us to pounce on the wrong clues when the right ones are under our noses. But it must never appear arbitrary from our side of the footlights. "Corpse!," in its current state, seems all contrivance -- a collection of moving parts masquerading as a diabolical engine.