- S4 E1 - Old, Unhappy, Far Off ThingsLewis learns that a serial killer he arrested thirteen years ago may be released on appeal. And Lewis and Hathaway area problems—can they put aside their differences and become as good as they once were?
- S4 E2 - Wild JusticeDuring an ecumenical conference in formerly Catholic St. Gerard's college, the poisoning of a female black American bishop triggers a series of bizarre murders.
- S4 E3 - The Mind Has MountainsThe ethics of a questionable drug on the Oxford campus of anti-depressants calls into account the questionable judgment of a psychiatry professor.
- S4 E4 - The Gift of PromiseWhen a retired MI5 official writes her best-selling memoirs, a chapter in her book about an unsolved IRA cold case causes consternation and ultimately several murders.