The Eternity Ring

Christopher Foyle comes home to a country that has changed its wars. Returning by ship after a year in the United States in 1946, he is stopped at immigration and taken to London, where British intelligence extends an invitation he cannot refuse. The audience has already been shown the world he is coming back to: the film opens on the Trinity nuclear test in the New Mexico desert in July 1945, the flash that announces the new enemy.

MI5 tells Foyle that British atomic research has probably been infiltrated by the Soviets and that the suspected channel runs through the physicist Professor Michael Fraser. Fraser's secretary is Samantha Stewart, Foyle's wartime driver, now married to Adam Wainwright and working under her married name; the service suggests she may have acted as a go-between. Foyle is reluctant to take the work, and accounts for changing his mind by recognising that any investigator other than himself might do Sam harm. Hilda Pierce, the wartime Special Operations Executive officer turned MI5, is at the table, and so is the operative Arthur Valentine.

Foyle works the case as a policeman, not a spy. A Russian agent, Aleksei Gorin, dies in what is presented as an accident, and Foyle quietly takes the dead man's keys and later lets himself into his flat; challenged by Sam on the legality, he answers that the security service itself has little regard for the law. The physical clue is a quantity of stolen fissile material, Uranium 233, taken from the Arnwell Atomic Research Facility and carried out in an ordinary vacuum flask.

The two strands meet, and the picture turns inside out. The Eternity Ring is not a real Soviet network. Foyle establishes that the supposed spy ring is a fiction used inside MI5, part of a power play by Pierce against her supervisor, Sir William Chambers. Separately, he establishes why Fraser took the uranium. Fraser was not working for the Soviets; he took the material because he believed that spreading the knowledge and means of nuclear weapons was the way to prevent their use. The service's enemy is an invention of its own politics, and the scientist's crime is an act of conscience. The dossier records that no source sets out the chain of events behind the poisoning that the episode's reviews mention, and that the account of the Russian agent's death is an accident as presented rather than as explained.

The episode's strength is that inversion. It replaces the whodunit with an espionage mystery in which the threat everyone has been chasing does not exist in the form the service believes, so the puzzle is about an institution rather than a culprit. Its weakness is density: the players arrive fast, and one reviewer of the set found too many coincidences and a story probably a bit too complex. Alongside the case run the domestic threads that carry the run, Adam Wainwright's Labour candidacy and the return of the former Hastings constable Frank Shaw from six years as a prisoner in Singapore to a wife and son who have changed in his absence.

This is the record that resets the series. It reunites Foyle, Sam and Pierce, introduces Valentine, re-introduces Adam Wainwright, and turns Pierce from an occasional sparring partner into the permanent MI5 figure who carries the thread to the finale. It ends with Pierce asking Foyle to join the service, and Foyle accepting. The transfer is a jolt that soon settles; the move to Cold War London breathes new life into the programme, and the cold open on the Trinity test declares that the world has changed.