An Agatha Christe mystery play is rather like a cake mix -- predictable ingredients in a neat package and difficult to ruin. "Verdict," Olney Theatre's first offering of the summer season, which opened last night, is something Betty Crocker would be proud of.

Let's face it, you either enjoy these corny concoctions or you don't. They are not the stuff of great theater, but they are entertaining. And while half the characters may seem to be stock issue off an assembly line, it is worth remembering that Christie invented many of these cliches.

It is a cardinal rule of reviewing a mystery not to give away the plot, because that would spoil what fun there is. "Verdict" has to do with an exiled German professor, Karl Hendryk, his disagreeable, wheelchair-ridden wife, her mysterious and prim cousin Lisa Koletsky, and the inevitable slovenly charwoman, who all live in a dreary flat in London in 1958. Enter Helen Rollander, a supremely self-confident ex-debutante (in this country she'd be called a Prep) who fancies herself in love with the professor, and by the end of the first act the murder has been accomplished.

The audience knows who has committed the crime, but not how the criminal will be discovered. In this play, which Christie considered one of her best, the uncovering of dreadful secrets is left to drift somewhat toward the end in favor of the expression of a few ideas, to wit: that the unending patience and kindness with which the professor treats his cranky wife, as well as everyone else including the murderer, can be as dishonest as plain old human meanness. "You put ideas first, not people," wails the willowy Lisa near the end.

This is the sort of play that is easily satirized. Nearly all the characters are introduced after doorbells ring, and some are announced, as in: "Oh, it's Inspector Ogden!" (we knew that inspector was lurking somewhere), or when the professor comes out with a line like "I cannot believe that innocence can go unrecognized," or when Miss Obnoxious ex-deb says, "I shall get my own way, you know" -- the corn is growing nearly as high as an elephant's eye.

The virtue of Olney's production is that it is all of a piece, consistent in its somewhat dated corniness and thus enjoyable for what it is. Director Leo Brady has wisely chosen to keep the play set in the late 1950s. Somehow those silly flat hats and tight skirts make the situation more believable.

The performers are almost all commendable, with Brenda Wehle as the starchy Miss Koletsky taking the honors for the best effort to avoid cliches, with Joanne Manley next for being so loathsome as the ex-debutante. Pat Karpen as the invalid wife, Irwin Ziff as the family doctor and Marlene Bryan as the nosy charwoman stand accused of mugging, and Bryan must stand trial for failing to bring even one new thing to a charter that has admittedly been cut with the Cockney charwoman cookie mold. In the central role of Prof. Hendryk, Bernie McInerney starts off convincingly as the excessively good professor, but gets increasingly hackneyed as the evening goes on. He anticipates embraces and other strenuously emotional moments, and thus dilutes the tension, in the end giving us more of a wimp than a befuddled idealist. John Neville-Andrews as Inspector Ogden is properly English (he's the only one in the cast with a legitimate claim to the accent), and Michael Nofstrand does a fine job with the part of a goofy student.

Verdict by Agatha Christie, directed by Leo Brady, scenery and lighting by James D. Waring, costumes by Pamela Tomassetti, with Bernie Mclnerney, Pat Karpen, Brenda Wehle, Joanne Manley, John Neville-Andrews, Irwin Ziff, Michael Nofstrand, Marine Bryan, John R. MacDonald and Jason Nunan.

At the Olney Theatre through June 28.