A growing number of therapists and self-styled healers believe homosexuality is a curable illness. By going undercover, and using secret filming, Dr Christian Jessen wants to discover the true nature of the "cures" by trying them on himself.
From old-school aversion therapy, carried out by the NHS, which required gay patients to sit in their own urine and vomit for three days, to modern techniques such as 'gay rehabilitation' and colour therapy, Dr Christian boldly undergoes as many 'treatments' as possible… and it makes for upsetting viewing.
We see the doctor being given given bottle of ipecac syrup to drink, to make him vomit for hours while looking at pictures of naked men and listening to tapes telling him he is "worthless," that being gay was evil and that he should want to have sex with women.
And, perhaps more shockingly, we see the popular television doctor speaking with teenagers outside an Evangelist church within the Bible Belt of America, hoping to find open-mindedness and instead being driven to tears.
Two teenage girls, aged 16 and 20, explained to Dr Christian that being gay "happens when bad spirits are inside you. Demons."
And their words clearly made an impact on the Embarrassing Bodies star, who cried: "Imagine being gay and being told that?"
"I think you should get me out of here, I’ve had enough," he then said to the camera crew, before walking away in visible shock.
Speaking with the Mail Online, Dr Christian explained: "I’d been doing reasonably well before then, keeping myself together and regarding all of this as just nonsense."
"Rather stupidly, I thought younger people must share different views and I hoped to see some open-mindedness. It saddened me deeply that this wasn’t the case.
"They don’t come up with these ideas themselves, they are taught them, and that’s incredibly worrying."
The eye-opening documentary reveals that a sizeable body of people believe homosexuality is a disorder which needs to be cured, forcing us to ask ourselves what these extraordinary practices tell us about 21st century attitudes towards homosexuality, medicine, psychology and sexual identity in both the UK and the USA.
Dr Christian, for one, says that he hopes the documentary raises awareness and helps the gay community feel more assured.
"I hope young, vulnerable people who perhaps may have considered these therapies will watch the programme and realise it’s all nonsense and they have absolutely nothing to be ashamed of."
Watch the trailer for the must-see documentary, which airs tonight on Channel 4 at 10pm: