Mystery plays have one advantage over their less-mysterious brethren. Even if they are not given a top-notch production, they can still keep an audience interested.

What the mystery play offers, more than anything else, is a plot, deftly manipulated. If the actors are moderately credible as they make their way through the labyrinth, we're willing to go along with them. After all, we're not there to discover new insights into human behavior. (The mystery play assumes that man is a duplicitous creature. What else is new?) We're there to learn how the age-old story unravels this time.

I think this may explain why the Round House Theatre's production of "Deathtrap," although acted with no particular expertise, is still reasonably enjoyable entertainment. Of all the recent Broadway mysteries, this one probably has the most adroit mixture of suspense and humor, and there's no disputing that author Ira Levin builds on a fruitful premise.

What would happen, he's wondering, if a once-successful mystery writer, now down on his luck and utterly devoid of inspiration, were to receive in the mail a script from an aspiring playwright, a script so good that it is almost worth killing for? Wouldn't it be possible to invite the young playwright over for drinks and a little professional counsel, and then . . . ?

Levin is nothing if not playful. The script in question also happens to be called "Deathtrap" and it bears more than passing resemblance to the events on stage. The characters, while presumably discussing the young man's play, are also commenting on the very play we're watching. The effect is not unlike the one you get when you place two mirrors opposite one another -- reflections within reflections within reflections.

The Round House production, directed by Jeffrey B. Davis, doesn't capture the self-mocking brio of the piece. It is acted in a low-key, rather straightforward fashion. Dion Anderson could use a little more quicksilver in the role of the burnt-out playwright and Lorraine Pollack would be more effective, as his wife, if she didn't strike quite so many poses. But Thomas E. Schall is perfectly plausible as the young playwright and Greta Lambert manages nicely as the next-door Dutch psychic, who drops in now and then with premonitions of what is about to happen next. (Her visions come close to the mark, but not so close that Levin ever spills more than he ought to.)

Although no play plays all by itself, mysteries do play more facilely than any other genre. The Round House actors don't lift "Deathtrap" to any remarkable heights. But, by the same token, they don't get in the way of the evening's machinations.

DEATHTRAP, by Ira Levin. Directed by Jeffrey B. Davis; set, Richard H. Young; costumes, Pamela MacFarlane; lights, John K. Gabbert; with Dion Anderson, Lorraine Pollack, Thomas E. Schall, Greta Lambert, David DiGiannantonio.

At the Round House Theatre through Nov. 1.