Amy Hartman’s “Mercy and the Firefly,” getting its world premiere at the Luna Stage in West Orange, is a harrowing and grimly funny bit of work. A nun on the run shelters a teenage killer, the Mercy of the title, fleeing from gang-ruled East Los Angeles to the gritty Pittsburgh of sister Lucy’s childhood.

No one is going to look for Mercy there, but Homestead harbors its own demons for Lucy, a nun who ranks on the piety scale somewhere between Helen Prejean of “Dead Man Walking” and one of the sisters from a convent on “The Simpsons.”

Hartman keeps melodrama at bay with humor. She doesn’t shy away from the jokes in introducing the play’s other two characters: Oliver, Lucy’s borderline junkie ex-boyfriend, and her cracked mother Viv, who dresses her own broken wrist in duct tape and a 7-11 bag and prepares (Symbolism Alert!) last meals for prisoners on Death Row.

As Lucy, Marcie Hartman gives us an affecting portrait of the would-be tower of strength who just can’t handle the pressure.

Christopher Daftsios plays Oliver with a rabbity grace.

Alicia Rivas has the hardest job as Mercy, the fish out of water who has to be by turns timid, threatening, lost, seductive and sweet. And fifteen. It’s a bumpy ride.

Andrea Gallo is alternately hilarious and moving as Viv. All four are well-served by Cheryl Katz’s no-nonsense direction.

“Mercy and the Firefly” is a good play with a really good play struggling to get out. There are some structural flaws. A friendship that blossoms between Mercy and Viv appears to happen during intermission. (Hey, no fair! We want in!) But what is on view is well worth your time.

Mercy and the Firefly
February 10 – March 13, 2011
by Amy Hartman
Directed by Cheryl Katz
Luna Stage, 555 Valley Road, West Orange

For tickets and information, click here.