In "Love From a Stranger," Frank Vosper's treatment of a tale by Agatha Christie, the Olney Theater is having some flawed fun. While the production occasionally falters and the play has its holes, the Olney has mustered a show of decidedly sinister style.

This is thanks in large part to David Cromwell who portrays the mystery's lady killer with a mix of threat and whimsy. He does it on the border of terror and humor, getting laughs increasingly edgy as the enormity of his acts unfolds.

The play, adapted from a short story titled "Philomel Cottage," marks a refreshing departure from the Christie M.O., wherein the audience must guess the villain's identity. Here, there's never a doubt that "the butler did it." The question becomes, rather, just what has he done?

The answer is revealed, mainly by hints and innuendo, in the story of comely Cecily. Having just won half of a 20,000-pound prize in the Irish Sweepstakes, she decides to rent out her flat in Bayswater so she can leave on a long vacation, and is about to break off a five-year engagement to the stolid Nigel Lawrence.

Against the better judgment of Aunt Lulu and Cecily's loyal roomie Mavis, the winner of the other half of the prize, she falls for the charming American who considers taking their flat.

"Who are you? What are you? Who are your people?" demands busybody Aunt Lulu of the brash intruder Bruce. But her inquiry comes too late. Bruce and Cecily get married and move to a secluded cottage.

The doings on stage are slow getting started, but quicken whenever Cromwell appears. Pat Karpen is suitably good-natured and vulnerable as Cecily, but isn't as sharp as Catherine Flye playing the sensible Mavis. Anita Dangler has her moments as Aunt Lulu, but does things a bit too broadly, from mugging to all manner of funny walks.

Cromwell -- swaggering with the bored self-assurance of a practiced cad, later staggering with the frenzy of a psychopath -- is a delight throughout.

One scene between Bruce and Cecily is particularly effective, the result of deft direction from Leo Brady. In it, she discovers her husband's true traits and he chases her around the living room. But the chase is leisurely, over polite conversation, which is at once creepy and amusing.

The production has its faults, such as sets that look tacky and occasionally work with embarrassing flimsiness; so does the play, mainly owing to Christie's penchant for a lot of odd clues that lead nowhere. But the evening on the whole is engaging -- and ominously entertaining. LOVE FROM A STRANGER -- At the Olney Theater through June 26.