For US agent Greg Squire and a global network of specialist undercover investigators, the mission is clear: track and catch serial paedophiles operating across the dark web with the sophistication of organised crime. Working across time zones with minimal resources, these officers spend months — sometimes years — burrowing into encrypted networks, piecing together clues across jurisdictions and deploying both cutting-edge digital forensics and old-fashioned detective instinct to unmask offenders who believe they are untouchable. Despite their remarkable successes, they know they are fighting a problem far larger than any one team can solve — but they keep fighting anyway, bringing light to the darkest places on the internet.

Directed by Sam Pirante for the BBC.