The season explores how a long-buried lie can devastate lives in the present. As Cassie and Sunny peel back the layers of the suspects' lives, they discover fractured relationships and deep-seated guilt.

When the body of 16-year-old Hayley Reid is found under the M1 motorway, Cassie and Sunny reopen a case from the turn of the millennium. Hayley had gone missing while on holiday in a seaside town in 1999, and the investigation soon centers on four close friends who were staying at a nearby holiday rental at the time.

(Kevin McNally): A successful TV presenter with a troubled son.

(James Fleet): A man who has struggled with homelessness and mental health issues.