Idris Elba returns as John Luther, the near-genius detective waging his own war on crime. This season Luther's biggest threat could be close to home, when it emerges that his own colleagues are willing to break every rule in the book to bring him down... Elsewhere it looks like Luther might have finally found love again, but is he ready to leave his past behind? Finally, when a vigilante killer embarks on a crusade to punish criminals, Luther is forced to confront his own sense of morality. Can he walk the line between right and wrong and do his job or has he finally met his match?