Half a million people in England turn to hypnotherapy every year, and Professor Kathy Sykes, who studies science and society at Bristol University, sets out to test whether any of it actually works. She watches a woman named Mandy have two teeth pulled with no local anaesthetic, only hypnosis; the patient reports almost no pain and her pulse rate stays flat while Sykes, watching, is nearly sick herself. Elsewhere the film follows Richard trying to quit smoking and Nicola trying to change her eating habits through hypnotherapy, tracking how much of the process is trance and how much is expectation. Sykes undergoes hypnosis herself to see what the state feels like from inside it, then takes the claims practitioners make, covering addiction, phobias, allergies, even breast enlargement, to researchers who can check them against evidence. Some hold up under scrutiny; the breast enlargement claim does not. The film keeps returning to one plain question: what, if anything, is actually happening in the brain when hypnosis works.