Crime author Tim Weaver sets out to understand how and why people disappear in the 21st century. As well as setting the challenge – is it really possible to disappear? - this week features conversations with two of the leading experts in missing persons investigations. Doctor Karen Shalev Greene is the Director for the Centre for the Study of Missing Persons and Sherri McAra is an analyst for the National Crime Agency’s Missing Persons Bureau. They work with Tim to walk through a missing persons investigation and lay out the surprising challenge of the task ahead.