Correction: In a previous version of this story, the identities of the twin brothers played by Thomas Keegan were incorrectly reversed. Peter is the brother who is the film and stage actor and Craig is the soldier who goes to Iraq. This version has been corrected.

Rachel Zampelli in Signature Theatre's production of ‘Dying City,’ a finalist for the 2008 Pulitzer Prize for drama. (Scott Suchman)

Does Peter, the rising star who invites himself into his sister-in-law’s flat one evening in the winningly understated “Dying City,” realize the depths of his insensitivity? Embodied by the strapping Thomas Keegan, Peter blithely mentions to taciturn Kelly (Rachel Zampelli) the dramas occurring on- and offstage in the Broadway revival of “Long Day’s Journey Into Night” in which he is currently appearing.

“The play is like being in a war,” Peter observes, and, of course, the remark could not fall on less-receptive ears. The first anniversary of the death in Iraq of Kelly’s scholarly, soldier-husband Craig — who also happens to have been Peter’s identical twin — has just occurred, and the shattered Kelly, a therapist, is far from over her grief and fury.

As playwright Christopher Shinn so slyly blends the dizzyingly contradictory feelings of his characters in this short but stirring play, Kelly has ample reason both to love and loathe her dead mate. And in Zampelli’s expertly controlled performance, a mask of fatigued resignation disguises the sense of betrayal and bewilderment raging within.

Signature Theatre is giving Shinn’s play, a finalist for the 2008 Pulitzer Prize for drama, a well-deserved Washington area debut. And aside from a tendency on Keegan’s part to hurry through some of Peter’s lines as if he were rushing to catch a bus, director Matthew Gardiner’s production supplies the right amount of restraint to a story loaded with resonant detail. The piece is a low-simmering psychological mystery, the sort of sophisticated evening that comes to a satisfying end without the burden of a thorough explanation.

Rachel Zampelli and Thomas Keegan in Signature Theatre's production of ‘Dying City.’ (Scott Suchman)

The question Shinn seems to be asking could be applied as fully to what transpires in Kelly’s New York apartment as to the turmoil in Iraq: Why do we do these things to each other? Though the play takes place in the early 2000s entirely in Kelly’s flat, efficiently realized by set designer Daniel Conway, “Dying City” goes back and forth in time, between the evening on which Peter shows up, and the final days that Kelly and Craig spend together, before he heads to a base in Georgia and ultimately to Baghdad.

The play, therefore, requires a quick-change artist (and a convenient number of cellphone calls that must be taken in an adjoining room) for an actor’s transformation from Peter into Craig and back again. This happens so many times during “Dying City” that one begins to suspect the dramatist counts on the audience to get a little confused. (“Now, he’s Craig! . . . Now, Peter!” a woman in Signature’s smaller theater, the Ark, kept stage-whispering to her seatmate in front of me.)

The disorientation reinforces our emotional allegiance to Kelly, who is subjected to mixed signals from both brothers, most significantly from Craig, who grows more distant and sullen as his date of departure nears. That the marriage is disintegrating even before Kelly realizes what’s happening is one of Shinn’s little ironies: As a therapist, she devotes her professional life to reading the warning signs in others’ relationships. Her antennae, it seems, don’t work over shorter distances.

If you enjoy looking for the mysterious convergences that do occur in both life and theater, you’ll particularly like that Peter is in “Long Day’s Journey,” during which the members of the Tyrone family obsessively pour out their grievances, revealing the extreme degrees to which they fixate on the wounds they receive at the hands of the others.

Duality presents itself everywhere. In “Dying City” — a phrase that uses to describe Baghdad, but refers, obliquely too, to the void in Kelly’s life — the pain is just as intense, but inflicted at times with more subtlety. When at a climactic moment Peter chooses to read to Kelly one of the many Baghdad e-mails from Craig that he has saved, she instantly realizes something’s amiss. (In this moment, those antennae work perfectly.) Not to give anything away, but could Peter possibly not have anticipated what happens next? Has the self-centered, maybe even vindictive, side of him laid a devastating trap?

Keegan has the chiseled look for the roles, and he and the sultry Zampelli have undeniable chemistry. His task here is, on a technical scale, far more challenging, and one suspects that a trace of an actor’s anxiety is evident as he attempts to play brothers almost as alike in temperament as in appearance. The result — by accident or even perhaps by design — is that he’s spitting his lines out at a clip that renders some of them difficult to process. Keegan clearly has this role in him; a spectator comes to feel that with a few more deep breaths on his part, the lines of this biting slice of homeland insecurity will really pop.

Dying City

By Christopher Shinn. Directed by Matthew Gardiner. Set, Daniel Conway; costumes, Frank Labovitz; lighting, Colin K. Bills; sound, Matt Rowe; original music, Lerman. About 65 minutes. Through Nov. 25 at Signature Theatre, 4200 Campbell Ave., Arlington. Visit or call 703-573-SEAT (7328).