When a car belonging to a former IRA man is pulled from the sea in Northern Ireland, DCI Tom Brannick finds an ominous postcard that connects to a shadowy case from the past: an assassin codenamed Goliath. Tom and his colleague DS Niamh McGovern investigate, but the community’s distrust of the police hinders their progress.

Tom reels from the discovery of three bodies buried 20 years ago—one of which may be his wife. Tom and Niamh interview Pat Keenan, the man who was kidnapped, but he won’t say who abducted him. Later, Tom confronts Adam Corrie, who was hunting Goliath.

As Tom scrambles to cover up what he’s done, Adam Corrie is deemed a missing person and his house becomes a crime scene. Tom searches for the woman in Pat’s car and finds a connection to Tori, his daughter’s medical school teacher.

Tom’s superior is arrested and interviewed, but Tori worries there’s not enough evidence for a conviction. Niamh remains unconvinced that they’ve finally caught Goliath and starts her own investigation.