The Talking Cure, National Theatre, London

On the psychiatrist's couch, you don't expect to be forced to lie back and think of England – or Austria, or Switzerland, or wherever the analysis is taking place. You have, by definition, other things on your mind. That hasn't, however, deterred some shrinks from taking deplorable advantage of their position of trust.

Only last month, a consultant psychiatrist from Suffolk was jailed for eight years for sexually assaulting a number of female patients.

Things were somewhat different in the early days of psychiatry. Intercourse with a patient was, to be sure, a professional taboo. There were, however, certain women who happily consented to sex with their analysts and then went on to make impressive careers in this nascent discipline.

One such was Sabina Spielrein, a young Russian woman whose problem was an orgasmic excitement at the idea of degrading punishment. She was referred to and cured by the Swiss pychiatrist Carl Jung, who was to become the favoured heir of the movement's great patriarch, Sigmund Freud.

In The Talking Cure, an engrossing new account of the story by Christopher Hampton, Jodhi May's splendid Sabina progresses from a shuddering, compellingly disturbed wreck to a self-possessed trainee who takes the sexual initiative with the crop-haired, twinklingly bespectacled and humanely convincing Jung, played by Ralph Fiennes.

She has a theory that this adulterous union with him is a victorious violation like the incest in Wagner's Ring that produces Siegfried, the pure hero. Hampton's play goes on to dramatise – a touch dutifully – how Jung came to regret his professional misconduct, not least because it helped hasten his ideological and personal split with Freud.

With its triple-tiered set that allows for psychologically charged differences in height, and with its shadowy, slightly Gogol-esque atmosphere, Howard Davies' production is properly haunting.

It is also, sadly, haunted by the sudden death during previews of James Hazeldine, the fine actor who was to have played Freud. That role is now heroically assumed by Dominic Rowan who may be too young and too tall but who, in also playing Freud's disreputable comic antithesis, Otto Gross, offers a neatly symbolic schizoid double as a repressive patriarch and a rebellious son. Anxious about the precarious position of his new discipline, Freud looked to the brilliant non-Jewish Jung to armour the movement against criticism from anti-Semites.

While showing that Sabina was a catalyst in their break-up, the play does not convincingly deny that this would have come about in any case. There's certainly dramatic diversion in the opportunities for mutual blackmail she afforded, but you don't feel that Jung's heretical belief in the collective subconscious or Freud's connection between the sex urge and the death instinct were decisively shaped by their contact with her.

Sabina Spielrein was a fascinating woman in her own right, later fighting to establish psychoanalysis as a discipline in the inhospitable Soviet Union. It's time we had a play angled to present Freud and Jung as catalysts to her career.