Agatha Christie's plays not only deal with murder, they usually get away with it.

Long-winded exposition, cardboard characters, contrived situations--such are the failings of all but a handful of her works for the stage. There is, however, always the promise of a last-minute surprise or two, and audiences can be remarkably tolerant if they sense a payoff at the end of the road.

Consider "Love From a Stranger," the mystery that Christie and Frank Vosper whipped up in 1936 from her short story, "Philomel Cottage." Last night, it opened Olney Theatre's 31st season, in keeping with an Olney tradition that says you lead off with dastardly deeds and save the respectable fare for later in the summer. If it came with any name other than Christie's attached to it, I suspect it would be hooted off the stage. Hooted? It probably wouldn't even make it to the stage in the first place.

This is surely one of the clumsiest pieces of writing the dear lady ever committed to paper. One of the slower-moving, too. It is not until the second scene of the second act that there is reason to harbor more than passing suspicions. The level of the dramaturgy, however, is not the real issue. What you want to know--and all that ever really counts in these matters--is whether or not the ending is any good.

Put it this way: True to her antiquated form, Christie delivers a bang-up finish. In the overly sedate staging of director Leo Brady, though, a fair amount of the bang has been muffled. Is this the moment, you well may ask, as the characters go into their life-and-death struggle, for so much polite sitting around?

The first act is devoted to extracting Cecily Harrington (Pat Karpen) from her impending marriage to a stuffy civil servant (Jack Hrkach) just back from the Sudan. "I want excitement. Life's been so monotonous up to now," complains Cecily. In no time, she allows herself to be captivated, courted and married by a charming adventurer (David Cromwell), who's come to rent her Bayswater flat.

Act II finds the newlyweds in a remote country cottage--no telephone, overgrown roads and servants who leave at nightfall. That alone is enough to alert even the most dense that trouble's on the horizon. But Cecily is too smitten with her husband to believe anything is amiss when the gardener unearths several hydrogen peroxide bottles in the flower beds. If her husband says that the basement is to be his preserve alone (for his camera equipment, he explains), why should Cecily doubt him? Perhaps because there'd be no play otherwise.

On the most elemental level--what's going to happen next?--Olney's production works okay, although it's hard not to think that with a little more style and ingenuity director Brady could turn this hoary claptrap into rousing claptrap. Fortunately, Karpen and Cromwell are adept at playing cat-and-mouse (I won't tell you which is which), and while they have been given relatively little to do in the way of stage business, they inject a goodly measure of psychological plausibility into events.

As Cecily's flibbertigibbet aunt, Anita Dangler performs her usual silly goose number. Perhaps when she is more secure in her lines, it will be funnier. The rest of the players--Catherine Flye, as Cecily's former roommate; Bernard Frawley, as the gardener; and Irwin Ziff, as a kindly country doctor--are all well cast to type. It is Brigid Cleary, however, who steals what there is to be stolen in the show, as the country bumpkin engaged as the maid. Cleary doesn't have more than two dozen lines, but nonetheless, she's cooked up a doozy of a character--eager as a beaver and twice as destructive with the china.

James D. Waring's two sets are serviceable enough, if you overlook the painted flats that pass for luxuriant shrubbery. On the other hand, Virginia Schwartz's costumes are excessively dowdy and the actresses should probably sue for defamation of figure. More attention to the little details certainly would help the cause. "Love From a Stranger" may not be bottom drawer, but it's getting close.

LOVE FROM A STRANGER. By Agatha Christie and Frank Vosper. Directed by Leo Brady. Scenery and lighting, James D. Waring; costumes, Virginia O. Schwartz. With Pat Karpen, David Cromwell, Anita Dangler, Bernard Frawley, Jack Hrkach, Brigid Cleary, Catherine Flye, Irwin Ziff. At Olney Theatre through June 26.