Like Peter Shaffer's superior drama Equus, Agnes of God introduces a self-searching therapist playing psychological connect-the-dots to solve a gruesome crime. Is the angelic, simple-minded yet somehow knowing young Agnes (see Killer Joe for another take on this familiar character) a murderer? Is she innocent by reason of insanity? Is someone—perhaps her Mother Superior—responsible? And how did a reclusive nun get pregnant, anyway?

Christine Mascitti plays the doctor, sharing her doubts, fears and biases with us (she's a lapsed Catholic, her sister was a nun). She's written as an obsessive chain-smoker, but this, like much from her, is only spoken, not lived: Her cigarette dangles and fizzles out, as perfunctory and unconvincing as her confessions. Since the doctor drives the story, this muted performance is hard to overcome—and Amy Chmielewski's mismatched costume for her only further distracts.

Hazel Bowers, making a welcome return to the stage, shines as the Mother Superior with more than a few secrets. She charms and bullies the doctor delightfully, trying to protect Melissa Lynch's lovely, fragile Agnes, who almost glows in her crisp white habit. Agnes proves wise as well as naive, either not understanding or artfully deflecting the doctor's probing. Perhaps she's just hiding the sinful, shameful truth—but what's causing those wounds on her palms?

Gregory Scott Campbell's production in the Walnut Street Theatre's Studio Five black box looks elegantly simple, albeit puzzling: A bright red carpet defines the doctor's office, overstating, along with Andrew Cowles' red-tinted lighting, the bloody birth that's described in detail. Agnes of God has greater aspirations that the production doesn't reveal: Lynch and Bowers make Agnes' plight fascinating, but the play's real power isn't as a "whodunit" (even if one of the suspects is The Almighty), but in the tortured doctor's unrealized dark journey back into her past—into the suffocation of her faith, and its possible reawakening.

AGNES OF GOD

Through May 21, Luna Theater Company, Walnut Street Theatre's Studio Five, 825 Walnut St., 215-704-0033 or www.lunatheater.org