Police close in on a robbery gang targeting people who buy and sell goods on Facebook Marketplace. The robbers are finally caught thanks to data captured by their electronic curfew tags. The data not only shows they weren't at home, it also places them close to the scene of the crime.

Also in this episode, a father whose daughter was murdered by her abusive and controlling ex-boyfriend launches a phone app he hopes will help keep other young women safe. And the parents of a hit-and-run victim help trace the driver who knocked their son down.

An ex-boyfriend throws acid into a young woman's face, then uses a series of disguises in an attempt to throw police off the scent. But number plate recognition cameras help prove he was in the area and responsible for the horrifying attack.

Meanwhile, a new online system is set up in Cumbria, enabling witnesses and victims of crime to upload video evidence and send it direct to the police. And a stolen Land Rover is recovered and returned to its owner in less than 24 hours, thanks to a hidden tracking device.