A family who paid the ultimate price for confronting moped thieves, prompting their close-knit community to help forensic specialists uncover evidence to unmask the knifeman.

Police receive a call: a 21-year-old university student has gone missing. Friends report that after being turned away from a club, Libby Squire was put into a taxi and hasn't been heard from or seen since, and it's very out of character.

Humberside Police launch one of their biggest-ever missing persons searches, while digital forensic experts trawl though CCTV. Libby is captured walking away from her home address. The team continue to track her movements and notice something concerning – a male figure mirroring her movements on the other side of the road. He then approaches her and leads her to a car. The car is tracked to a playing field. Libby is not seen again, and a witness reports hearing a chilling scream around the same time.

Libby's disappearance prompts a number of women to come forward with disturbing reports of being watched, followed, or exposed to, by a man. Forensic experts link a number of the women's reports – leaving them in no doubt there is serial sexual predator in Hull, but with Libby still missing the team can't prove if she has fallen victim to the same offender.

Then a body is found in the Humber Estuary, and scientists use microscopic forensic evidence to unlock the case.