A young NHS hospital doctor called “Ruth” is suspended because she is suffering from depression. She does not tell her employers that she is also hearing a voice in her head, telling her to kill herself. To do so would mean dismissal and probably get her sectioned. She turns to Rufus May, an unorthodox NHS psychiatrist who rejects the use of medication. He believes such voices are full of meaning and that patients should engage with them. He talks to the voices himself. He talks to Ruth’s.

Without medication Ruth becomes temporarily deluded. She is working in a care home for the elderly and thinks that the fish tank in the sitting room is a heart monitor hooked up to one of the residents. Many people would feel terrified of having this otherwise charming young woman treating them in hospital. May wants to get her back on the wards.

This is the disturbing, but challengingly real story of The Doctor Who Hears Voices, the latest film by Leo Regan. His previous subjects have included drug addicts, racists and, more recently, in Scars, an explosively violent criminal. Regan likes to reach the corners of humanity that normal documentary film-makers don’t.

He spends months getting to know his subjects, winning their confidence, coaxing them to open up. If his subjects refuse to be identified, he uses the transcriptions of their conversations as scripts and reconstructs their interviews, talking to professional actors. Jason Isaacs gave a chilling performance as the thug in Scars. In this film Ruth Wilson is equally convincing as the troubled subject.

But, as Regan explains when we meet in a vegetarian café in North London, this film has been far more complex than Scars. Intrigued by reports of a pressure group called Mad Pride, he had gone to Channel 4 proposing to do a film exploring the stigma of mental illness. The organisation led him to May and the initial plan was simply to follow him in his work.

“The sections of the film where it’s just me talking to Rufus are straight documentary,” he says. “I spent 18 months with him and saw him working with 48 people. We documented some stories that we thought it wasn’t right to show.” Ruth’s story stood out because it encapsulates the moral dilemmas for doctor and patient alike in an extreme form, but, for obvious reasons, she would only cooperate on condition that she could not be identified.

Regan spent a lot of time talking to the real Ruth on her own. “Rufus wasn’t always comfortable with that,” he says. “He was afraid I might be provoking her in a way that could make things worse.” These conversations are re-enacted by Regan and Wilson using his transcripts. The strangely multilayered reality of the film is complicated even further when Wilson playing Ruth, tells Regan (playing himself) that the voice in her head doesn’t like either May or himself and often mocks them in a way she finds very funny.

A third form of drama-doc hybrid comes when Regan reconstructs encounters between May and Ruth at which he was not present. This time there are no transcriptions, and May and Wilson must improvise using May’s notes of the meetings and what the real Ruth has told Regan she recalls of them. It is, Regan says, his most ambitious project yet.

A Londoner currently based at a hospital in Bradford, May is a former mental patient himself, though he hates such negative terminology. When he was 18 he had paranoid delusions, Regan tells me. “He was found walking naked in the street, and he thought the KGB had implanted a gadget in his chest.” Now free of his illness and happily married with two children, May hates the diagnosis of schizophrenia and the stigma it brought.

But he also knows his methods are controversial and that some of his colleagues regard them as irresponsible if not downright dangerous. “Agreeing to do the film was a huge risk for him,” says Regan. “He had to agree that the film would continue wherever it took us.” At one point, in fact, it seems his treatment may have provoked a dangerous crisis in Ruth. “I rushed up to Bradford overnight to talk to him when he was feeling at his most vulnerable,” says Regan. “I wanted to film him in that moment.”

Regan is shocked by the idea that the film might be seen as too favourable to May. “I pushed him hard all the time,” he says, “challenging what he was doing. I hope that comes across. I had no idea myself how things were going to work out for Ruth.” Further balance is provided by interviews with Trevor Turner, an expert on schizophrenia who is convinced of the efficacy of medication, intrigued by May’s methods but also extremely wary of them.

Underlying the whole film is the moral issue of whether psychiatric patients should lie about their conditions. May himself had to do this and tells Ruth to do so when she goes for her appraisal. In this respect Turner is a surprising ally, agreeing that the stigma of diagnosis is so great that he would omit it from an application form if he were applying for a job.

Inevitably, this film raises far more questions than it can possibly answer. But it shines a powerful light on a condition that, for the sufferers and their families, can be, literally, a matter of life and death.

The Doctor Who Hears Voices, Mon 21 April 2008, Channel 4, 10pm

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