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Graham Seymour was the deputy of MI5. His father, Arthur, had been a member of British Intelligence during and after World War II and had been pro-Arab. Seymour later became the director of MI6.
Seymour is notified by Gabriel Allon that some unidentified operation is being planned against American targets in London.
Seymour discussed possible reasons for Grigori Bulganov's disappearance with Gabriel Allon. He initially resisted attempts by Gabriel to acquire more information. After Chiara Zolli was abducted, he led the MI5 effort to assist the Office.
Seymour was asked by the prime minister, Jonathan Lancaster, to find Madeline Hart. He, in turn, asked Gabriel Allon to do the field work.