The Miseducation of Cameron Post review: Tale of conversion therapy packs punch

She runs a conversion therapy camp called God's Promise in an appropriately secluded stretch of American countryside with her brother and prize exhibit, the Reverend Rick (John Gallagher jnr), who proudly claims she's enabled him to overcome his attraction to other men.

Chloe Grace Moretz's Cameron Post winds up here after she's caught making out with her girlfriend on high school prom night and desperately wants to get out and go home. She's long since accepted the fact she's gay. Her only regret lies in getting caught by the wrong person – the boy who took her to the prom.

Although this sounds like the set-up for a teenage variation on One Flew Over the Cuckoo's Nest, it's a much quieter and more subtle take on institutional wrongheadedness – and all the better for it. Apart from an implacable belief in their own infallibility, Dr Lydia and Cuckoo Nest's Nurse Ratched have little in common. Straitjackets and ECT are not part of God's Promise. Dr Lydia prefers to lead her charges to recognise their sins for themselves. Nonetheless, the resulting damage is just as serious.

Emily Danforth's novel, on which Desiree Akhavan's film is based, is set in small town Montana in the 1990s, but its many fans have given her reason to believe that conversion therapy camps still exist in evangelical pockets of the US. Cameron has grown up in one of them, having been adopted by her Fundamentalist Christian aunt and uncle after the death of her parents in a car accident. She loves them and they love her, but in this case, love doesn't guarantee wisdom. They're sure she can be "cured".

Moretz takes an oblique approach to all this, feeling her way into the role with a watchful intelligence that keeps all cliches at bay. Cameron seems genuinely curious about the aplomb with which Dr Lydia dispenses her brand of pseudo-psychology and after gathering a few tips from two other camp rebels, she settles down to keep out of trouble by manipulating the system in an attempt to persuade everybody the treatment is working.

The climax, when it comes, is devastating, its effect amplified by the understatedness of everything that surrounds it. Dr Lydia is not guilty of malice, but sincerity. Insulated by ignorance, she's impervious to commonsense. Akhavan may have a light touch but it packs a punch.