Sectioned, BBC Two

“Are you looking forward to Christmas?” was always going to be a difficult question. Anthony looked forward to spending it with his daughter and grandchild – as long as he kept taking the medication that allowed him to stay out of hospital. Andrew should have had a happy gathering lined up, except his latest bout of mania had seen him leave the family home. Richard was wrapping presents.

Sectioned, Ben Anthony's documentary about mental health first shown as part of BBC Four's Out of Mind season, told a story that was very necessary, with a conviction, engagement and empathy that came right out of the best traditions. Of course there was much that we didn't see: the vast work the director had to put in to get it there, the effort expended in earning the full trust of all involved, months of persuading a local health authority to cooperate at all. It opened with Anthony checking that his three patient-subjects were aware of what they were doing, and happy with it. What we also didn't see was that they and their doctors had that rarest thing in modern documentary: final-cut approval.

So we don’t know whether the director just wasn’t around when moments of conflict on the wards came up, or if he excised intentionally any that did. Either way, the result was a surprisingly gentle picture. Andrew, a retired pathologist, seemed to retain the most self-awareness that he was in a film, as to a lesser extent did Anthony – no coincidence, perhaps, that their chances at the end of it looked brightest. Richard, by contrast, was as raw as they come. Scarred, literally, by a pan of boiling oil thrown at him when he was six, then sexually assaulted at school, at 34 he had been in and out of treatment for 19 years, and opened up the most to the director: from revealing where he hid the odd bottle of vodka from the authorities, to running heartrendingly through his collection of childhood photographs (we never saw his family members on screen). But even at moments when he came across as the gentlest of souls, could we completely disregard a nagging doubt that he was in some way performing for the film, just as he admitted performing to the review board that would take the next decision on whether to keep him confined or not?

I couldn’t help being reminded of Paul Watson’s documentary on alcoholism, Rain in My Heart – the same relentless search for access, the same gruelling empathy with those who give their trust to the director, the same urgency of the final message. There was even a scene in which Andrew, now back home from hospital though clearly still in considerable confusion, played the piano, a spooky echo of another Watson film, Malcolm and Barbara – Love’s Farewell, his story of the impact of dementia on a middle-class marriage. It's the ultimate compliment to say that Sectioned belongs in the same tradition. Rain in My Heart saw the death of two of Watson's four characters by the end of the programme. Anthony closed Sectioned with a final line of text that played with our expectations subtly (and finally benignly), revealing that Christmas has passed, and that his three protagonists are still going (though certainly not going strong). Just don’t, whatever you do, call it a shot-in-the-arm for the industry.

• Watch Sectioned on BBC iPlayer