Frozen, a controversial play by British dramatist Bryony Lavery, is playing through April 1 at the Walnut Street Theatre Studio 5. It's a disturbing drama about a murderous pedophile, a psychiatrist who studies the brains of serial killers, and the mother of one of his victims. Isis Productions's three actors — Kristen Quinn as Agnetha, the doctor; Allen Radway as Ralph, the killer; and Renee Richman-Weisband as Mrs. Shirley, the bereaved mother — are bravely performing intense and difficult roles on a small stage, in intimate proximity to the audience.

The drama begins when Nancy Shirley is waiting for her 10-year-old daughter Rhona, sent on an errand to her grandmother's house, to come home. If she is Little Red Riding Hood, then Ralph is the big bad wolf.  We watch him seduce the girl: "Hello," creepily inflected about a dozen different blood-chilling ways. Eventually he is caught, his shed containing the remains of seven girls is discovered, and he is imprisoned for life.

Mrs. Shirley tries many solutions for her grief, but eventually, 20 years later, in the play's strong second act, she goes to visit Ralph to tell him she forgives him.  She shows him family photographs, trying to make him understand the loss and the horror he has caused. But, as he tells psychiatrist Agnetha (Quinn), who befriends him, "The only thing I'm sorry about is that it's not legal." "What's not legal?" "Killing girls."

Frozen raises troubling questions about "crimes of evil and crimes of illness," asking whether those who commit hideous deeds — child rape and murder — are to be blamed or whether they themselves are the victims of dreadful childhood abuse, with brain-damaging injuries that alter the capacity to function as moral human beings. Ralph's reenactment of his father's punishments is a frighteningly powerful scene, demonstrating what the doctor believes is the difference between "sin and symptom." Which brings up a resounding question: Is moral responsibility a viable concept?

Lavery's first act is almost entirely in direct address, so each of the three characters speaks only to us and each has his/her own station on stage.  This isolation is purposeful but deflates the drama. Richman-Weisband's attempts at an English accent often undermine the character's extreme emotions. Neill Hartley directs a very strong second act, when both Radway and Quinn soar as they finally interact and reach a new level of understanding.

Scandal broke over Lavery's head when it was discovered that she had taken much of her play's material from the life and work of a real psychiatrist. That scandal threatened to derail both the play and the playwright's career, yet that very plagiarism has lent a weird sort of authenticity to the psychiatric theories on which Frozen rests.

The play's title suggests "the Arctic frozen sea that is the criminal brain" and suggests what happens when that sea melts.