The stories of the families of people who were killed on camera by members of a British Islamic State terror cell dubbed the Beatles. ITV's global security editor Rohit Kachroo spends time with Bethany Haines, Marsha and Carl Mueller, and Diane Foley and hears about their fight to bring the murderers to justice, while Spanish hostage and survivor Marc Marginedas provides first-hand testimony of life in the Syrian prison where he was held alongside many of those who later died.