Unruly police detective, DCI Charlie Hicks is deeply suspicious of his new partner DI Elaine Renko. But their first investigation exposes a secret that will connect them for life.

Hicks and Renko track down a murderer who has developed a dangerous obsession with the Hard Sun conspiracy. Meanwhile, Renko recruits an ambitious journalist to further expose the truth, which leads to her becoming a target for security services as the government does everything in its power to ridicule the story as a hoax.

A murderer confesses to a Priest that he wants to challenge God by targeting those who help others, and Grace makes Hicks an offer.

London is under siege as Blackwood’s reign of terror escalates, and Father Dennis becomes a public pariah. Hicks deduces where Renko has hidden the Hard Sun flash drive.

The murder of a jogger has dire personal implications for Renko but she pretends it is a routine investigation. Hicks makes a startling revelation about the Alex Butler murder.

An internet killer is targeting people who believe they wish to die and Mooney thinks he knows how to trap him. Renko and Hicks change tactic and bring the fight to Grace's door.