UCOS reinvestigate the death of Annabel Tilson, a diplomat in the Foreign Office whose body was discovered in a frozen river on Christmas Day 2008. Having suffered a miscarriage and been demoted at work, Annabel's death was presumed to be suicide. This view is shared by her devastated husband Eddie. However, Annabel's eccentric twin sister Minnie is convinced that she was murdered to cover up a conspiracy within the Foreign Office.

Eddie is profoundly deaf, and working with an interpreter prompts Detective Superintendent Standing to start learning sign language himself in the hope of reconnecting with his deaf cousin Danny. Meanwhile, Pullman clashes with Annabel's old boss Peter Hammond and starts to wonder if Minnie's conspiracy theory could be right after all. Show less