While monstrous and seemingly indiscriminate killings become ever more audacious and public, Luther and new recruit D.S. Catherine Halliday (Wunmi Mosaku) are confounded by a complex tangle of leads and misdirection that seems designed to protect an untouchable corruption. As the body count rises, and gangster George Cornelius (Patrick Malahide) applies his own pressure, can Luther catch a killer and save his own neck?