Neil LaBute's provocative post-9/11 play The Mercy Seat, now at the Lyric Stage in Boston promises much, but delivers little. LaBute sets up a terrific "what if" and manages to perk us up with a nifty surprise twist as things grind down, but in between, he doesn't fully explore the possibilities of his premise, nor even hold our attention some of the time.

The play begins with an unanswered ringing cell phone. Set in the pre-dawn hours of September 12, 2001 in a well-appointed downtown loft commanding a view of where the twin towers used to be, Ben Harcourt (Robert Pemberton) slumps motionless on the sofa. He's unshaven, his face unwashed, still dressed for work the day before. Faced away from the silent TV screen with its endless loop of disaster footage and seemingly oblivious to the ringing phone in his hand, he appears to be a man alone, one of the many individuals whose lives were put on hold by the unspeakable events of the day before.

But wait, he's not alone. Abby Prescott (Paula Plum) returns from a successful mission to get supplies: a 6-pack of batteries, a nice Havarti and a box of Carr's crackers. As she shakes the soot from her clothes and wipes the ashes from her face, she tells Ben about following a young woman who was taping flyers to every available surface in the neighborhood, another addition to the growing wallpaper of photos, each with a phone number and plaintive plea: "Have you seen this person?"

We soon learn that Ben is, in fact, one of the missing. As luck would have it, he'd stopped in for a "quickie" with Abby on his way to an appointment at the World Trade Center. We soon learn he's trying to decide between letting his wife and young daughters know he's okay and "wiping the slate clean to start over."

We also discover in due time that Abby and Ben's affair has been going on for three years, she's twelve years older than he is, the Persian rug is not "faux" and that she is Ben's boss, having beaten him out for the promotion. We never find out exactly what they do or why they show no concern for their fellow workers, including those in the downtown branch that presumably no longer exists. Nor do we have a clue as to why Abby is also ignoring her ringing phone even though it's four in the morning and she's a well-compensated manager at a Manhattan firm touched by the tragedy.

In the course of the endless 90-minutes or so with these two, we are also informed that Ben never looks at Abby when they have sex, never talks on the phone in her presence for fear of "sharing" the other part of his life (clearly they never worked in adjacent cubicles) and subscribes to the cliché of saying he's going to leave his wife, but never doing it. What we don't know is why Abby would want this man if their first full day alone together is any indication of future performance. Nor do we glean why Ben's home life is so miserable that he'd toy with the idea of never being able to contact his children again.

LaBute provides little "color commentary" to flesh out these characters, only a series of sudden recriminations that don't even build upon each other. On top of that, Ben and Abby's inability to do anything nice for each other, including offering any physical comfort whatsoever prevents them from just collapsing together on the sofa. The apartment is neat as a pin with no evidence of its proximity to the disaster nor the fact that two distraught people have been holed up there watching television for 24 hours, so Abby remains on her feet a great deal of the time with little to occupy her.

The decision by director Eric C. Engel and scene designer Brynna Bloomfield to have audience on all four sides of the playing area contributes to Abby's moving about for no reason and manages to tie the hands of the lighting designer as well. Eleanor Moore doesn't have a row of big loft windows through which to illuminate the eeriness of the outside world and reinforce the time of day.

What's more, the placement of the meager two rows of additional seating to add audience members to the fourth side leaves them fully lit and in full view of everyone else. That location may be a little too stuffy for seating judging by the number who nodded off.

The original New York production in December 2002 (which was directed by LaBute) sounded so interesting, I can only surmise that Sigourney Weaver and Liev Schreiber brought it to life either by channeling previous creations or infusing their own eccentric personalities. Too bad LaBute wasn't working with Boston troupers Plum and Pemberton that first time out. While they were giving it their all, he might have gotten to work on a healthy rewrite to flesh out his fascinating ideas.

The Mercy Seat now through April 17th at the Lyric Stage Company, 140 Clarendon Street (YWCA Building, 2nd floor) in Boston's Copley Square. Performances are Wednesdays & Thursdays at 7:30 PM, Fridays at 8:00 PM, Saturdays at 4:00 & 8:00 PM, Sundays at 3:00 PM and Wednesdays at 2PM (Mar 24 & Apr 14 only.) Tickets are $22 to $41 depending on performance and seat location with a Student Rush of $10 on sale 1/2 hour before each performance. Tickets are available at the Lyric Stage box office (617) 437-7172 or online at www.lyricstage.com.

Photo: Spiro Veloudos

Be sure to check the current schedule for theatre in the Boston area.