DC Harford discovers a new suspect who doesn't have an alibi. Cath reveals new information about the night of her party. Hardy and Ellie get a match on the male DNA found on Trish.

Cast: Julie Hesmondhalgh, David Tennant, Olivia Colman, Jodie Whittaker, Charles Babalola, Simone Ashley, Lenny Henry, Sarah Parish, Charlie Higson, Richard Hope, Georgina Campbell, Hara Yannas, Jim Howick, Hannah Millward, Andrew Buchan