Karl Barclay, a medical student missing for six years, comes to the attention of the Cold Case team when his DNA matches that of a corpse also found six years ago in Hales reservoir, in the fourth story of the new series of the crime drama. As Eve discovers signs of torture on the corpse, Boyd wants to know why the case hadn't rung alarm bells earlier, and becomes increasingly suspicious that there has been a cover up.
Boyd learns of the family tension that erupted when Karl's evangelical Christian mother, Lisbetta, discovered his conversion to Islam. Meanwhile, Karl's secret stash of Islamic material seems to point to a terror plot. Via her all-knowing ex-colleagues in counter-terrorism, Sarah confirms that Karl was on a watch list.
An increasingly unhinged Lisbetta attacks a member of the public, insisting he killed her son. After Boyd tells Gideon and Jakob that Karl was on the police's watch list – suspected of terrorist activity – Lisbetta admits to Gideon that she "had to stop" Karl...