Suspect: Richard & Natasha

Susannah is shocked when DS Richard Groves, her former lover, doesn't seem to take her concerns about Jon's murderous discourses seriously, and soon realises that he's hiding something. She locks horns with Richard and his wife Natasha and manages to get a lead - the name of a police officer who reported a girl saying her friend was murdered. Mystery, starring Anne-Marie Duff, Tamsin Greig and Ben Miller