When 16-year-old Becky Watts went missing in 2015, it sparked a major police investigation to find the teenager. But only two people knew where she was - her stepbrother Nathan and his girlfriend Shauna. Here, Susanna Reid gains unprecedented access to police interview room footage to reveal the strategy and skill involved in catching Becky's killers. There is also first-person testimony from some of the officers, and Susanna talks to Becky's family about the impact of this appalling crime