A mysterious man known only as Six arrives in a remote village in Scotland. Meanwhile, PC Rachel Heggie awakens from a nightmare about being abused as a child before heading out on patrol. Whilst on patrol, Heggie witnesses Caesar’s car strike Six. Six disappears, and Heggie arrests Caesar. At the police station, she is greeted by Sgt Macready, who charges Caesar with reckless driving. Heggie radios PCs Mundie and Warnock to find the victim before taking Caesar to the cells, where he joins his teacher, Mr. Beswick, who beat his wife.
Mundie and Warnock, having found Six, bring him to the station, mute and covered in scratches and carrying only a notebook with lists of names. Heggie phones Dr. Hume to examine him. However, upon examination, Hume experiences a flashback and attempts to kill Six for supposedly knowing what he did. Hume is placed in a cell, and Six is identified as an old man who died over 20 years ago. Six makes several references to Old Testament justice before Macready suffers a flashback and angrily has Heggie and Mundie place Six in cell six. Mundie also has a flashback to beating a prisoner with Warnock.
Macready has Mundie and Warnock go to Hume’s house to find his wife before also driving to his own home. Heggie researches the names in Six’s notebook, who are all deceased criminals. In the cells, Six, who displays supernatural powers, quizzes Beswick for the true reason why he beats his wife. Unsatisfied with Beswick’s answers, Six induces him to attempt suicide. Caesar alerts Heggie, who allows Hume to examine Beswick, who dies after claiming to know who Six is. Elsewhere, Mundie and Warnock enter Hume’s house and find he has murdered his family. Macready hurriedly disposes of a dismembered body in his fridge along with the body of his boyfriend, who he beat to death in a jealous rage.
Hume helps Heggie carry Beswick upstairs, but he attacks her when she learns about his family. After restraining him, she returns him to his cell. Six reveals he knows about Heggie’s abuse, and, when she leaves, he offers Caesar redemption if he confesses to a hit and run accident that involves a young girl. Mundie and Warnock return and drag Hume to interrogation, ignoring Caesar’s attempt to confess. As Hume confesses to killing his family out of nihilistic boredom with his career and a desire to unlock the secret of immortality, Warnock talks briefly to Six and has a flashback to staging the suicide of the man he and Mundie had beaten. Enraged, Warnock rushes to interrogation and brutally kills Hume in front of Heggie and Mundie. Heggie is threatened into remaining silent.
Caesar confesses to Heggie about hitting the girl, but Six says it is too late, as the girl died two minutes earlier. Heggie has another flashback and remembers that Six distracted her childhood abuser, allowing her time to escape. Six informs her Mundie and Warnock are planning to kill her. Heggie fights Mundie and Warnock, who nearly succeed in strangling her, until the sudden return of an insane Macready, who has wrapped his torso in barbed wire and is quoting verses from the Bible. He shoots and kills Warnock, then wounds both Heggie and Mundie as they barricade themselves in the cells. While Macready sets the building on fire, Six makes references to his true identity, of losing an argument with “an old friend” about human sin, and being an outcast, like Heggie. He offers to help her, but only if she asks him to. She refuses.
While escaping, Mundie slips and cuts her throat on a broken window, and Macready wounds Heggie before killing Caesar. Heggie throws Macready’s fuel can at him, which he accidentally shoots. Mortally wounded in the explosion, he quotes another bible verse, after which Heggie responds “Amen” and kills him. Heggie escapes the police station, and Six appears, unburnt, with his notebook, which he reveals has everybody’s name inside. He crosses off every name except Heggie’s and condemns the others to Hell for their various sins. Six reveals he is in love with Heggie and offers her a place at his side to seek vengeance on the guilty; she agrees, and they kiss.