Don’t get too comfortable in your seat during Ira Levin’s “Deathtrap,” now being presented by Winterport Open Stage at the Wagner Middle School. Just when you think you might be getting a handle on the plot, the person who was killed in the last scene comes walking through the door. Dead doesn’t necessarily mean dead in this play.

We’re not talking ghosts here. We’re talking plot device. Kill him in one scene, and in the next scene reveal that he was part of a master plan to steal money and love.

If it sounds like a good idea for a thriller drama, that’s because it is — to the tune of more than 1,700 runs on Broadway. Levin, who also wrote “Rosemary’s Baby” and “Dial M for Murder,” uses “Deathtrap” to tell the story of Sidney Bruhl, a washed-up mystery writer who agrees to tutor Clifford Anderson, a young playwright who was a student at a seminar. Seems the fledgling writer has penned a play (called “Deathtrap”) that shows a great deal of promise.

Because Sidney’s wife, Myra, is worried about waning finances, she implores Sidney to insist on getting credit for any work he does on Clifford’s play. But instead, Sidney strangles Clifford. And off we go on a string of murders and zigzagging plots.

Reed Farrar, artistic director at WOS, has fun with this twisted set of circumstances, which are a substantial challenge for such a small theater. Farrar keeps the action rolling along, however, and comes up with a show that’s part melodrama, part sitcom. He has chosen to underplay the homosexual part of Sidney and Clifford’s relationship, which steals some intensity from the play’s action, but by no means compromises the show as a whole.

The five actors in the cast work hard at developing characters that are both quirky and plausible. Don Sleight, as Sidney, maintains a drawing-room stature, and Suzanne Hall, as Myra, is a blithering bundle of brandy-soaked nerves. She’s so antsy and gaspy that it’s not such a bad thing when she gets killed off.

Darald Libby-Haines, as the ambitious Clifford, starts out like a sweet idealist, and then bonks the audience on the head with a much more complex characterization. The transformation is graceful and impressive.

Rose Marie Sweeney, as the clairvoyant neighbor Helga Ten Dorp, steals the show. Granted, she has some of the best lines in this rather wry play. But her scenes are filled with a natural cleverness and a hilarious animation that light up the stage.

Kevin DeBeck nicely takes on the smallish role of Porter Milgrim, the Bruhls’ lawyer who has a plot of his own going.

The murder scenes in this show are very effective. It’s not just the blood, but the suspense that this cast is able to build for the audience. That’s not to say there aren’t a few hokey moments, but it all works for this drama-within-a-drama piece. The production could benefit from speeding up the lines a bit, but there’s much to wince about in this show. And when you’re talking murder mysteries, that’s a good thing.

Winterport Open Stage will present “Deathtrap,” 7:30 p.m. Aug. 7-9 at Wagner Middle School on Mountain View Drive in Winterport. For information, call 223-2501.