Marcella is devastated when the killer targets a 6-year-old girl, but then a DNA match gives the police their strongest lead in the case. Meanwhile, Williamson investigates Andrew Barnes' death.
Stars: Toby Wharton, Anna Friel, Nina Sosanya
Director(s): Jonathan Teplitzky
Writer(s): Hans Rosenfeldt, Nicola Larder, Mark Greig