A thriller is a most particular thing. It's like a mystery play with the rules broken. Mystery plays do have a sensible scheme: a crime, lots of clues, several people with possible motives, red herrings and a final denouement scene in which some splendid person such as Hercule Poirot, Jane Marple or Jessica Fletcher takes us kindly through the whole business, identifying the criminal, explaining the motive and tying up all the loose ends.

A thriller, such as "Night Watch," currently offered by The Castaways Repertory Theatre, is a more reckless affair.

A crime or crimes may have been committed: The leading lady, Elaine Wheeler (Shirley Attanaro), claims to have seen two murdered bodies in a window of a vacant building; then again, we only have her word for it. When the police investigate the scene, they find no bodies and everything covered in several inches of undisturbed dust. No one believes Wheeler, but can we trust any of them?

After all, she is being given medication, but what is it? Is it what she thinks it is? What about the person who administers it (the imperturbable Ali Boylston plays Blanche Cooke) and is so eager to have her take it? Should the fact that she is a nurse reassure or alarm us? Clearly, she's suspect.

What about Wheeler's husband, John (a somewhat over-stressed interpretation by Chuck Smith) -- does he love her, or is he trying to drive her away? He says he's sending her to Switzerland to cure her insomnia. Does he have other interests? Is he suspect? Of course.

And what about that maid? Helga (played with an astonishing selection of Eastern European accents by Denise Oppenhagen) seems devoted to her mistress but suddenly wants to visit a dying relative and needs cash fast. Also suspect.

There's a flamboyant, intrusive neighbor (James Howard plays the jolly Curtis Appleby) who seems awfully interested in the objets d'art the Wheelers have. But then, so is the patrolman (Gregory Powell is refreshingly real and effective as Patrolman Vanelli, without resorting to any acting hijinks). And Lt. Walker (Dale Peters, who returns to the stage after many years absence, without disgrace) dismissed Elaine Wheeler completely. Naturally, all the above are suspect.

Then there's even a psychiatrist, Dr. Lake (played in a low-key fashion by Cathlyn Macaulay, who nevertheless must not forget to project). By the time she enters the play (Act II), she could be in league with any one of the others.

A man who owns a nearby delicatessen, Sam Hoke (a small role played with vigor by Bill Kight), arrives and Elaine Wheeler screams. He looks just like the man she saw dead in the window. Was he posing as a dead man? If so, who is he in cahoots with, now that he's suspect?

Playwright Lucille Fletcher knows the thriller -- this one played on Broadway in 1972 -- and while we have been energetically working out every possibility, calculating every clue, she gives us the ending we could never have anticipated. But it does, of course, make perfect sense, and we all say to ourselves, "I knew that."

The well-thought-out and careful set is by Scott Bailey (who also acts and directs for Castaways). The clever costumes are by Ellen Young.

Terri Caretti Young does a neat, clean job of the direction.

Some sound effects offstage are almost too faint to discern, although the shots near the end are deafening.

A good thriller is a pleasant way to pass less than two hours, especially around Halloween. And this one has a free gift thrown in.

It is not really necessary for people to act all that well in thrillers -- adequate will do. Shirley Attanaro as Elaine Wheeler is something we could not expect. She is doing stunning work almost all the time she's giving a thrilling performance.

"Night Watch," The Castaways Repertory Theatre, The Godwin Middle School, Darbydale Drive, Dale City. Through Saturday. For tickets and information, call 330-9814.