With time running out, AC-12 attempt to unmask 'H',the Fourth Man (or Woman) commanding the network of corrupt officers behind the murder of Gail Vella. But sinister and powerful forces appear intent on orchestrating a cover-up.

Cast: Martin Compston, Peter De Jersey, Kelly Macdonald, Laura Elphinstone, Adrian Dunbar, Vicky McClure, Perry Fitzpatrick, Anna Maxwell Martin, Amy De Bhrún, Shalom Brune-Franklin, Rosa Escoda, Maria Connolly, Claire-Louise Cordwell, Nigel Boyle, Owen Teale