Luther remains tortured by the past a year on from the death of estranged wife Zoe and friend Ian Reed's betrayal. Alice is in a psychiatric unit and Ripley is stuck in uniform whilst Luther shuts himself away with dusty files working on cold cases. Martin Schenk ends up offering Luther a place working in the new Serious and Serial Unit and he ends up accepting. The team finds themselves being immediately faced with a surreal and nightmarish case. They have to find a man who hides behind a Punch mask as he brutally kills his victims.