Prologue: Oxford 1967 'I am Piles!' The source of this enigmatic statement was a bespectacled, white- haired man in his sixties, wearing a drab grey sports jacket with a pouch of tobacco protruding from one pocket. His right hand fiddled with the pouch's contents, while the left played with a pipe that refused to light. He was five feet seven and a half inches tall, as I would later learn from MI5 files. It was a September morning in 1967. The encounter took place in the hallway of 12 Parks Road, a Victorian house in north Oxford, an ample and slightly downatheel building that could easily have doubled for a vicarage in an Agatha Christie novel. In those days 12 Parks Road was home to the university's Department of Theoretical Physics. I was a nervous new graduate student on my first day in Oxford. I had walked up the gravel drive opposite the University Parks, and entered: as the front door swung closed behind me, the thud echoed back from the rafters two flights of stairs above.

I found myself in a silent, gloomy hallway. Linoleum covered the floor; a stairway of dark wood led to upper landings; imposing oak doors were all shut. A musty smell hinted at decades of decay. The walls were painted ochre, their only decoration a large board that listed the scientists who had offices there. Against each name was a tag, which indicated whether the individual was 'IN' or 'OUT'. It appeared that everyone was 'OUT'. Alone in this apparently empty mansion, I was wondering what to do, when from the subterranean depths of the basement a man appeared like a mole from its burrow. He greeted me in a strong central European accent, with stress on every word: 'Can I Help Yo?' Taken aback by this strange encounter with (I assumed) the janitor, I replied 'I don't think so', to which the apparition made the bizarre reply that he was 'Piles'.

Flustered, I then realized that he had actually said 'I am Peierls':PiUrls - Rudolf Peierls, the father of the atomic bomb, and one-time mentor to Klaus Fuchs, the most damaging spy of modern times. More than three decades after he had fled Nazi persecution, Peierls' Germanic cadences remained. Breathless at failing to recognize the head of Theoretical Physics, I stuttered, 'I am Frank Close.' I was about to explain why I was there when Peierls proffered his hand as if my arrival was the highlight of his day. 'Plizzed to Mit Yo,' he said firmly, nodding his head in welcome as he led me into his office. At my high school in Peterborough, entering the inner sanctum of the head's study had been a journey of dread. But in Oxford, Peierls ushered me kindly into a new world. Here I was welcomed as a new member of an international family of scientists.

I was twenty-two years old, the same age as many of the physicists who had designed and built the atomic bomb - and the same age as Klaus Fuchs when he first settled at university in England, like Peierls a refugee from Hitler.