Since the previous episode of Foyle’s War, retired DCS Christopher Foyle has been to America to bring an old antagonist, Howard Paige, to justice for murder. Justice has apparently been served, since Paige has committed suicide. In other news, Foyle’s trusty former driver, Sam, is now married to her sweetheart, Adam Wainwright. They are living in London, where Sam is the secretary to Professor Michael Fraser, a nuclear physicist.
The new episode opens with a flashback to July 1945: the test of the first atomic bomb in New Mexico. Among the observers are members of the British atomic bomb project, Max Hoffman and Professor Fraser, along with Fraser’s wife, Helen, who later falls ill from exposure to fallout from the test.
Flash forward to a year later: Foyle has arrived back from his quest to catch Paige and is immediately whisked away to MI5 headquarters. His mission: determine if an alleged spy cabal called the Eternity Ring is real or a deception peddled by a Soviet defector. Foyle’s M-like handler is his old acquaintance, Hilda Pierce, formerly with the top secret Special Operations Executive. Her boss is Sir William Chambers, who convinces Foyle to take the assignment by showing him a picture of Sam supposedly passing nuclear secrets to a known Soviet go-between named Marc Vlessing.
Foyle is as conscientious as ever about his assignment and doesn’t tell Sam that she is suspected of spying. In pursuit of information, he has lunch with her, meets Professor Fraser, and later has dinner with the professor and Hoffman. Then Hoffman’s sources alert him that Foyle is working for the Security Service. When Hoffman tells Fraser, the professor fires Sam.
Sam is furious that Foyle wouldn’t level with her. For his part, Foyle is beginning to feel that he is a pawn in someone’s ineptly played chess game. He determines to get to the bottom of it with Sam’s help.
Meanwhile, Professor Fraser and Hoffman go to a meeting at Britain’s uranium-enrichment facility, where, coincidentally, a quantity of bomb-grade material suddenly goes missing. Unaware of the uranium heist, Foyle and Sam search Vlessing’s apartment while he is out. They are interrupted by machine-gun-toting British agents in radiation-hazard gear. A quick sweep with a Geiger counter shows that the apartment, Foyle, and Sam are “hot.” They have stumbled on the missing uranium! Shortly after, Vlessing is murdered.
By now, Foyle has concluded that two unconnected spies are at work. He confronts Pierce, his handler, for her duplicity in forging the photo of Sam and Vlessing to get his participation in a ruse to snare her boss, Sir William, whom Pierce suspects of being a double agent. The Eternity Ring is her hoax to smoke him out, which she does, since he breaks his cover to meet with a Soviet contact to learn the truth about the ring.
Foyle has also surmised that Professor Fraser is behind the uranium heist, since Foyle observed the same tea thermos in the professor’s house and Vlessing’s apartment. Suitably lead-lined, this was the hiding place for the bomb material, which the professor wanted to distribute to other nations in the idealistic belief that this would make the world safer. He is also motivated by the personal tragedy of his wife’s radiation sickness.
In other action, Adam will be the Labour Party candidate for Parliament in his constituency, and Foyle’s former constable Frank Shaw is having a challenging time returning to police work after six years in the Army.