The foreign affairs programme Unreported World (Channel 4) is now in its 15th year and 30th series, and still determinedly digging away in the trenches of human misery, bent on bringing into the light some of the worst overlooked abuses of which our sorry excuse for a race of supposedly sentient, civilised beings is capable.

This week, it was the turn of South Korea’s traffickers in people – homeless, often mentally disabled or otherwise vulnerable individuals in a country with no welfare state to speak of – to the salt and seaweed farms on the distant, perilously isolated Shinan Islands.

Chung Jie-hong, whom a childhood bout of meningitis had left with severe learning difficulties, was trafficked 30 years ago when he was 19. He is one of hundreds who have been freed from conditions that more than meet the UN definition of slavery since inspections of the farms began last year. As he tries on the colourful socks patterned with cartoon characters that delight him, he tells presenter Marcel Theroux how he was passed from farmer to farmer, island to island, was beaten and abused, and of the suicide attempts he made.

Theroux meets one of the farmers who remembers “employing” him. Jie-hong was “just a very bad man”, he insists. “His eyes are very scary.” He never beat him, the boss insists. “I don’t know how to reconcile this disparity,” says Theroux, with nice understatement.

Theroux meets several other former slaves – and one who is still working for the farmers he has taken to calling “Mummy and Daddy” – all drawn from the 80% that the director of a rescue centre for trafficked people reckons have learning difficulties. He talks to a “recruiter” scouting a nearby park for new victims. This man doesn’t target them, he insists, but, “We could take the mentally disabled by accident. There isn’t a full health check”.

Once again, Unreported World succeeded in compressing several lifetimes’ of human misery into one diamond-hard nugget and holding it up to the light. I wish it could illuminate what to do next.