David Marshall Grant first attracted attention for his off-Broadway sleeper hit “Snakebit,” a far stronger outing than both “Current Events,” his second play, and “Pen,” his third. With its generic living room set, mom-dad-and-son cast, and supply of one-liners, “Pen” feels like a familiar sitcom. But in this sitcom the nagging Jewish mother is in a wheelchair, so it feels awkward and unkind when we laugh at her. These sharp jabs might have been the point of a play about resentment of the ill and dependent. But in “Pen,” they’re mere distractions in a forced march toward pat sitcom resolution.

The problematic figure in this splintered family is not its chaotic son, Matt (Dan McCabe), who fails to mention his shoplifting conviction on his college application. Nor is it the buffoonish father, Jerry (Reed Birney), who has divorced his wife and gotten engaged to a young secretary. In the play’s terms, the troublemaker is Helen (J. Smith-Cameron), who has allowed the deterioration of her body (due to multiple sclerosis) to turn her into a self-centered shrew.

Helen is the type to refuse to give her son the car keys so she can trap him at home with her on a lonely night. She seems constitutionally incapable of going five minutes without nagging Matt, whose teenage contents are under sufficient pressure to explode at any moment. The big blowup comes at the end of the first act, when Matt confesses that he’s stolen his mother’s favorite pen – the only one she can use to write with while lying down – to write an application to an out-of-town college. He hurls the pen at his mother, yelling, “I give up. Take your pen. Take my legs. Go on, walk!”

To this point, the sitcom has been chugging along in the familiar way. It’s almost as if “Pen” was being made with the cookbook open – combine one nagging mom, one dad in midlife crisis, one out-of-control teenager; add major revelation every 15 minutes. But suddenly Mr. Grant goes off the recipe, dousing the whole thing in a heavy splash of magical realism.

Because presto! Now Helen can walk, and it’s Matt whose legs are stiff and useless. I’m not sure what bothered me most about this fantastical leap – whether it was the absolute lack of preparation for it, or the casual, quick way in which it was announced (just before the act break).

“Pen” might have recovered from one far-fetched suspension of disbelief, but hard on the heels of mom walking comes a truly unconvincing scene in a bar. Helen (in a blond wig and heels) runs into Jerry (her husband for two decades) and manages to prevent him from guessing her identity (while inches away) with the aid of dark glasses and a Southern accent. At this point, Mr. Grant exhausts the audience’s supply of good will.

Much as the fine actors try, they can’t compensate for such confused, cluttered writing. Nor can they get free of the unpleasant atmosphere created by Mr. Grant’s facile treatment of Helen’s MS. (Once she can walk, all her bad qualities melt away.)

The director, Will Frears, does little to smooth out the many rough edges. His staging of the miracle scene is so fast and vague that some theatergoers may miss its point the first time around. Working without a curtain, he uses jarring, odd techniques to get his characters on-and offstage; the handicapped walk off, while the able-bodied Jerry glides into the wings while seated at a sliding restaurant table.

As a playwright, Mr. Grant can write decent jokes, when he’s not listening too hard for the laugh track to kick in. “Pen” has stretches of authentic, engaging dialogue. But if he has the tools to make funny and moving realistic dramas, why has Mr. Grant written nothing more than a sitcom with clunky magic tricks?

Until April 16 (416 W. 42nd Street, between Ninth and Dyre Avenues, 212-279-4200).