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A dying man writes a letter to a senior civil servant who commits suicide. SAS Major Harry Maxim is seconded to security at No 10. One of his tasks — to look after nuclear strategist Professor John Tyler. A Czech defector with information worth killing for — is there a connection?

The Czech defector starts a 'mole' hunt. The KGB are interested in the missing letter. Charley Farthing wants his day in court to denounce Professor Tyler, and Harry Maxim has a hard time with them all.

In France, Agnes and Maxim meet the other surviving member of the group in the photograph. In Luxembourg, Professor Tyler arrives for the European Conference — while Harbinger manipulates and Harry discovers there are no rules in their business.