WATERTOWN -- ''Frozen" is a play that speaks about the unspeakable. There is the unspeakable grief of a mother whose 10-year-old daughter has been murdered. There is the unspeakable horror of the murder itself, combined with the sexual abuse that most likely preceded it, along with the unspeakable terror of what the girl must have been going through.
Bryony Lavery gives voice to all these anguishes in her play, which doesn't make for easy listening on the audience's part. But what sets ''Frozen" apart from less ambitious dramatic forays into this dark territory is that she also gives voice to the inarticulate mumblings and musings of Ralph Wantage, the killer, not so much to garner sympathy for him as to gather information about the mind of a murderer.
Ralph himself isn't able to make much sense of his life and sickness as he grunts obscenities left and right. That's left to the other two characters in the play: Nancy Shirley, the mother, and Agnetha Gottmundsdottir, the psychiatrist, both of whom have their own issues and demons.
''Frozen" was warmly greeted first in England and then in New York, where it played two years ago, and it's easy to see why. Lavery's intelligence and compassion are evident in every line, and she has created characters that give good actors a chance to shine.
Shine they do at the New Repertory Theatre, where the play is receiving its Boston-area premiere. In showing how the unbearable can become bearable, Nancy E. Carroll knows when to step on the gas and when to hit the brakes as the depressed mother. Bates Wilder won't make anyone forget Brian F. O'Byrne's Tony-winning performance as the murderer, particularly in reaching for a British accent, but his more simian portrayal is no less frightening, and his befuddlement is perhaps even more realistic. Adrianne Hewlett brings some welcome comic relief to the part of the psychiatrist, the least convincing of the three characters.
All three of these people are souls on ice. The mother and the murderer are stuck in a frozen mind-set of grief or rage that keeps them from moving forward. The psychiatrist's self-doubt has a numbing effect on her as well.
Director Adam Zahler and the design team smartly capture the wintry feel of the piece. Richard Wadsworth Chambers's set features a layer of white sand spreading from the characters' three chairs -- which are onstage throughout -- toward the audience. Carroll makes the most use of the sand, thrashing around in grief and burying her dead child's stuffed lion with the coffin. Karen Perlow's lights and Jeffrey Alan Jones's sound and music add to the atmospherics.
But the production highlights the play's weaknesses as well as its virtues. The ''frozen" metaphor is extended to the breaking point. And the constant presence of the three characters -- they retreat to their chairs when they have no dialogue -- sometimes works, particularly in highlighting the interrelatedness of their acts, and sometimes doesn't. We could use some relief from these forlorn people.
And despite the provocativeness of ''Frozen," the play seems a bit pat. Every thematic hair is in place, which makes for a tight script and a psychologically convincing explanation of what drove Ralph crazy. Given the murkiness of the material, though, a little mess might have made for more artistry; witness Martin McDonagh's ''The Pillowman."
Nevertheless, this is the humanistic play Lavery set out to write. In detailing the differences between evil and illness, she makes a case for understanding rather than vengeance, without lessening our empathy for the victims. That's an accomplishment that the New Repertory production brings to the fore with great dignity and detail.
Ed Siegel can be reached at email@example.com.