A new witness turns Trish's case upside down. Katie discovers a new link between the case and a key suspect. Beth tells her supervisor about Trish's case.

Cast: Kelly Gough, Olivia Colman, David Tennant, Hannah Millward, Julie Hesmondhalgh, Mark Bazeley, Georgina Campbell, Jim Howick, Hara Yannas, Sarah Parish, Jodie Whittaker, Sunetra Sarker, Lenny Henry, Charlie Higson, Arthur Darvill