Anyone intending to go to Welsh National Opera's revival of Janacek's Jenufa should consider whether or not to shut their eyes 15 seconds or so before the end. Just as we reach the final curtain Katie Mitchell's production - up to that point a model example of claustrophobic, naturalistic theatre - acquires a symbolist apotheosis. The set's back wall rises to reveal a paradisaical garden, in which a small boy holds out his arms towards the infanticide Kostelnicka, who totters towards him in ecstasy.

The scene's meaning is painfully unclear. Mitchell fails to tell us whether she is tacking on a religiose ending in which Jenufa's murdered child forgives his tortured killer beyond the grave, or whether she is flashing forward to an imagined future, in which Kostelnicka, released from prison, has become a contented step-grandmother to Jenufa's child by Laca.

The scene squares uneasily with Janacek, who trawls the depths of the human soul with Dostoyevskyan intensity, though he leaves open the resolutions attendant on moral and spiritual awareness. At the end of Jenufa we know neither the result of Kostelnicka's trial, or whether Jenufa and Laca even have a future together. Mitchell's interpolation is too decisive, and undermines the excellence of what has gone before.

This is a shame, because musically this is one of the finest Jenufas you will ever hear. Conductor Charles Mackerras, today's greatest Janacek interpreter, exposes the score's throbbing vitality and the fierce tensions generated by its rhythmic complexities, melodic angularity and raw sonorities. The cast gives the impression of living the work rather than performing it. Susan Chilcott's Jenufa is a fragile, sensual creature, her voice soaring with passion and anguish, even as her body contorts with pain and exhaustion. Suzanne Murphy's Kostelnicka, middle-aged and primly glamorous rather than old and rigid, is terrifying as guilt slowly eats away her mind and dignity. Her revelation that Jenfua's baby made no sound as she killed him is harrowing. The men are wonderful too, with Nigel Robson's bullish Laca, more violent and impulsive than most, pitted against Peter Wedd's vapid, yet sexy, Steva. Except for Mitchell's awkward final lapse, the whole thing is overwhelming.

· At the Hippodrome, Birmingham (0870 730 1234) on Thursday, then touring.