945:- A group of scientists observe the testing of an atomic bomb at Los Alamos in the New Mexico desert. A year later Foyle is approached by Hilda Pierce,now working for MI5. Her boss Sir William Chambers explains that a Russian defector,Gorin,has supplied them with news of the Eternity Ring,a group of Soviet sympathisers bent on betraying Britain's atomic secrets. Sam is now working as a secretary to Professor Fraser,one of the suspects who was present at Los Alamos and Foyle meets up with her,being introduced to Fraser and his colleague Max Hoffman,a Polish communist who escaped Nazi persecution. Both work at Armwell Atomic Research Station. Hoffman connects Sam to Foyle and she is sacked but when uranium is stolen from Armwell and the thief murdered she joins Foyle in investigating the Eternity Ring - as well as helping husband Adam stand for election as a Labour MP.

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