Dependable as we like to think Agatha Christie was, there were surely days in her long career when the clockwork plot just didn't come together and her characters, like the old lady in the shoe, had so many motivations they didn't know what to do. "The Hollow" seems to have been born of one such day.

Revived Tuesday night by Olney Theatre as the opening show of its 30th season, this 1951 mystery play nearly talks itself into the ground before the mandatory murder is committed. Then it talks itself a couple of feet deeper before the crime is solved. As usual, Christie leads an audience down the garden path, but the three-act trip is without suspense, and waiting at the end is not her customary surprise turnabout--just the garden's unsurprising edge.

Hard-core Christie fans may welcome the standard conventions--the country estate filled with weekend guests, the imperious butler, the daffy aristocrats and a police inspector who wants to know, "What were you doing at the time of the crime?" while his sidekick makes eyes at the maid. Christie has a right to her cliche's; after all, she invented them. But Olney's production--directed with more duty than flair by Leo Brady--doesn't take much enjoyment in them. Hoary is the adjective that springs to mind all evening long.

As a novel, "The Hollow" had Hercule Poirot in pursuit of the murderer, but Christie never felt he belonged in the tale and eliminated him from the stage version. The mantle of eccentricity passed to Lady Lucy Angkatell (Paddy Croft), a dizzy creature whose mind resembles a butterfly's flight and whose speech consists largely of non sequiturs. When the police inspector brings up the vexing matter of the derringer in the bottom of her egg basket, she is positive there is a logical explanation if only she could find it in the clutter of her memory.

"I wonder what it could have been," she says, bewildered. Then, turning the tables, she asks the inspector with all the good will in the world, "Why should I have taken that pistol?" It's a neat switch, although the rest of "The Hollow" is decidedly less original.

The plot revolves around an ancestral estate, clearly worth preserving, and a bounder of a doctor (Richard Vernon), clearly worth eliminating. That doctor is already juggling the favors of a slavishly adoring wife (Pat Karpen) and a lithe mistress (Joanne Manley) when an old flame (Sharie Valerio), now a famous movie star, sashays grandly onto the premises, multiplying the thwarted passions. What Christie's characters don't do for love, they are apparently willing to do for real estate.

If it were played with more relish, "The Hollow" might squeak by as a summer's divertissement. But the Olney cast is a mixed blessing. Croft has a good grip on Lady Angkatell's quicksilver mind, and the performance ought to get even better as the four-week run wears on. Manley's cool blonde presence is reminiscent of Hitchcock's heroines, and Debra Cerruti endows a poor cousin with convincing principles. Vernon is all too bland a cad, though, to merit much venom, and Valerio's tarted-up actress seems to have sprung from porno loops, not Hollywood blockbusters. The others more or less make do with stereotypes.

For several seasons now, Olney has led off its season with a Christie mystery, and devotees of her tidy acts of mayhem have made her that theater's all-time box-office champ. The repertory is not inexhaustible, however. Besides the crack of firearms, the noise you hear emanating from the stage in the Maryland suburbs these nights is the bottom of a barrel being scraped.

THE HOLLOW. By Agatha Christie. Directed by Leo Brady; scenery and lighting, Joseph St. Germain; costumes, Pamela Tomassetti. With Pat Karpen, Paddy Croft, Michael Rothhaar, Richard Vernon, Joanne Manley, John MacDonald, Sharie Valerio, Debra Cerruti. At Olney Theatre through June 27.