In the early hours of 21 April 2007, on the dark and treacherous Bwlch Mountain Road in south Wales, a woman’s body is discovered on the carriageway. At first glance, it appears to be a tragic road traffic accident. The victim, 21-year-old Jenna Watkins, is lying near a crashed car, but as police and forensics experts begin their work, it emerges that Jenna did not die in a collision.