Thelma and Louise were the first women to pick up guns this summer and defend themselves against the seemingly unavoidable brutality of men. Then came “Terminator 2” and “V.I. Warshawski,” films in which women fire a few feminist rounds to keep themselves or their children safe. Now, at Acadia Repertory Theatre in Somesville, there’s “The Unexpected Guest,” an Agatha Christie whodunit which, in the very first scene, puts a gun in Laura Warwick’s hands and poses her as the murderer of her husband Richard, a cruel and brutish invalid.

The gun passes through many hands before we definitively know who the killer is, but we’re never quite as intrigued by the murder motive as when we suspect the culprits to be Laura or Richard’s elderly mother, Mrs. Warwick, whose disappointments with her son are many.

Christie’s commentary in this play about the difference between how men and women handle crimes of passion is perhaps the most interesting sentiment in an otherwise rather boring script. As it turns out, the real murderer commits a crime tantamount to the ones of today’s pistol-packing mother figures in the cinema, but with an interesting twist. That fact alone makes the outcome of this story worth waiting for.

Otherwise, this show is somewhat of a disappointment, especially considering that Acadia Rep’s summer murder mystery is often the highlight of the season. Suspense nuts will find plenty to enjoy in this show, but there’s no mistaking that this production simply lacks the dynamism typical of a Christie drama. Guest director Wayne Loui has only scratched the surface of the fun this play can be, and replaces the humor and quick wit with a more serious and melodramatic style. And with all that high drama happening on stage, the play progresses more like a family funeral than a fun-and-games thriller.

In the lead roles of Laura and the “unexpected guest” Starkwedder, Susan Horner and apprentice Darren Campbell (respectively) show intermittent skill, but finally offer rather bland characterizations of people in sticky circumstances. In general, the show needs more volume and swifter delivery.

But despite some uneven acting and missed comic opportunities, several of the actors contribute some lively characterizations. Kathleen McInerney speeds up the pace with her vibrant portrayal of the housekeeper/secretary Miss Bennet. Alex Bruehl, who in real life is a sophomore in high school, captures the spirit of the deranged young Jan Warwick.

Christian Brandjes as the valet Henry Angell is smashing, and Eugene J. Tierney as the investigator keeps his eyebrow raised with a goodly amount of curiosity and arrogance.

John Erickson might have been better cast in one of the lead roles, but gives a fine performance as Laura’s paramour Julian Farrar. And Ken Stack, as Richard Warwick’s corpse, is impeccably inert.

Costumes by Katharine Tyson and set design by Stack are pleasant. However, the snippets of music that indicate scene changes, and the fog that floods the stage in the last scene, are hokey.

“The Unexpected Guest” will be performed 8:15 p.m. Tuesday-Sunday through Aug. 11, and 2 p.m. Aug. 11 at the Acadia Repertory Theatre in Somesville. For information, call 244-7260.