Just when you thought BBC3 could do nothing right (think Are You Hotter Than Your Daughter? Snog, Marry, Avoid?) it comes up with a documentary so beautifully sensitive and incisive that it makes you think it may after all not me the colossal waste of the license fee it appears to be the rest of the time.

Sam and Evan: From Girls to Men follows a year in the lives of two young people as they grow-up from being children into being adults – moving in with each other and getting their first job – but at the same go through gender re-assignment.

Coming from two entirely different worlds – Evan growing up on a deprived council estate in Rochdale and Sam coming from a prosperous household (complete with maid) in Surrey – Sam makes the decision to give up a place to study at Oxford and to forgoe the security and privilege of his upbringing to move in with Evan's family and endure daily abuse from their unenlightened neighbours.

Evan's parents seem to go on an even bigger journey than the two boys:  with dozens of pictures of her former daughter hanging on the wall in the background, his mother Kath gives and eloquent and incredibly moving account of her shock  upon hearing that Yvonne felt like she  couldn't grow up to be a woman – she talks as if her daughter had died – and her subsequent happiness at seeing her new son fall in love.

Kath and the rest of the family welcome Sam with open arms and despite the hostility and bleakness of the place they live, the two lads are able to make their transition in a loving, supportive atmosphere that is a joy to behold.

Uplifting though the story is, the director nevertheless leaves us in no doubt about the enormity and difficulty of the change Sam and Evan are going through. Making the choices the lads are is not simply a question of adopting a curious 'alternative' lifestyle along the lines of being a nudist or a vegan – as popular perceptions of transsexualism appear to assume – but a matter of living a fulfilling life, or hardly living at all. As Sam says: “If I had a choice between living as a girl or not living at all, I would choose just to not live.”