It happened over a glass of champagne. John Nicholas Rede Elliott's father, Sir Claude Aurelius Elliott, OBE, was headmaster of Eton (England's grandest public school), a noted mountaineer, and a central pillar of the British establishment. Sir Claude knew everybody who was anybody and nobody who wasn't somebody, and among the many important men he knew was Sir Robert Vansittart, chief diplomatic adviser to His Majesty's government, who had close links to the Secret Intelligence Service (SIS), better known as MI6, the agency responsible for intelligence gathering abroad. Nicholas Elliott arranged to meet "Van" at Ascot and, over drinks, mentioned that he thought he might like to join the intelligence service.

Sir Robert Vansittart smiled and replied: "I am relieved you have asked me for something so easy."

"So that was that," Elliott wrote many years later.

The old boys' recruitment network had worked perfectly.

Nicholas Elliott was not obviously cut out to be a spy. His academic record was undistinguished. He knew little about the complexities of international politics, let alone the dextrous and dangerous game being played by MI6 in the run-up to war. Indeed, he knew nothing whatsoever about espionage, but he thought spying souhnded exciting and important and exclusive. Elliott was self-confident as only a well-bred, well-heeled young Etonian, newly graduated from Cambridge University, with all the right social connections, can be. He was born to rule (though he would never have expressed that belief so indelicately), and membership in the most selective club in Britain seemed like a good place to start doing so.

The Elliotts were part of the backbone of the empire; for generations, they had furnished military officers, senior clerics, lawyers, and colonial administrators who ensured that Britain continued to rule the waves—and much of the globe in between. One of Nicholas Elliott's grandfathers had been the lieutenant governor of Bengal; the other, a senior judge. Like many powerful English families, the Elliotts were also notable for their eccentricity. Nicholas's great-uncle Edgar famously took a bet with another Indian Army officer that he could smoke his height in cheroots every day for three months, then smoked himself to death in two. Great-aunt Blanche was said to have been "crossed in love" at the age of twenty-six and thereafter took to her bed, where she remained for the next fifty years. Aunt Nancy firmly believed that Catholics were not fit to own pets since they did not believe animals had souls. The family also displayed a profound but frequently fatal fascination with mountain climbing. Nicholas's uncle, the Reverend Julius Elliott, fell off the Matterhorn in 1869, shortly after meeting Gustave Flaubert, who declared him "the epitome of the English gentleman." Eccentricity is one of those English traits that look like frailty but mask a concealed strength; individuality disguised as oddity.

Towering over Nicholas's childhood was his father, Claude, a man of immovable Victorian principles and ferocious prejudices. Claude loathed music, which gave him indigestion, despised all forms of heating as "effete," and believed that "when dealing with foreigners the best plan was to shout at them in English." Before becoming headmaster of Eton, Claude Elliott had taught history at Cambridge University, despite an ingrained distrust of academics and an aversion to intellectual conversation. The long university vacations gave him plenty of time for mountain climbing. He might have become the most celebrated climber of his generation, but for a kneecap broken by a fall in the Lake District, which prevented him from joining Mallory's Everest expedition. A dominating figure physically and psychologically, Claude was nicknamed "the Emperor" by the boys at Eton. Nicholas regarded his father with awed reverence; in return, Claude alternately ignored or teased his only child, believing, like many fathers of his time and class, that displaying affection would make his son "soft" and quite possibly homosexual. Nicholas grew up convinced that "Claude was highly embarrassed by my very existence." His mother avoided all intimate topics of conversation, according to her only son, including "God, Disease and Below the Waist."

The young Elliott was therefore brought up by a succession of nannies and then shunted off to Durnford School in Dorset, a place with a tradition of brutality extreme even by the standards of British prep schools: every morning the boys were made to plunge naked into an unheated pool for the pleasure of the headmaster, whose wife liked to read improving literature out loud in the evenings with her legs stretched out over two small boys while a third tickled the soles of her feet. There was no fresh fruit, no toilets with doors, no restraint on bullying, and no possibility of escape. Today such an institution would be illegal; in 1925 it was considered "character-forming." Elliott left his prep school with the conviction that "nothing as unpleasant could ever recur," an ingrained contempt for authority, and a hardy sense of humor.

Eton seemed like a paradise after the "sheer hell" of Durnford, and having his father as headmaster posed no particular problem for Nicholas, since Claude continued to pretend he wasn't there. Highly intelligent, cheerful, and lazy, the young Elliott did just enough work to get by: "The increased legibility of his handwriting only serves to reveal the inadequacy of his ability to spell," noted one report. He was elected to his first club, Pop, the Eton institution reserved for the most popular boys in the school. It was at Eton that Elliott discovered a talent for making friends. In later life he would look back on this as his most important skill, the foundation of his career.

Basil Fisher was Elliott's first and closest friend. A glamorous figure with an impeccable academic and sporting record, Fisher was captain of the First XI, the chairman of Pop, and son of a bona fide war hero, Basil senior having been killed by a Turkish sniper at Gaza in 1917. The two friends shared every meal, spent their holidays together, and occasionally slipped into the headmaster's house, when Claude was at dinner, to play billiards. Photographs from the time show them arm in arm, beaming happily. Perhaps there was a sexual element to their relationship, but probably not. Hitherto, Elliott had loved only his nanny, "Ducky Bit" (her real name is lost to history). He worshipped Basil Fisher.

In the autumn of 1935 the two friends went up to Cambridge. Naturally, Elliott went to Trinity, his father's old college. On his first day at the university, he visited the writer and poet Robert Gittings, an acquaintance of his father, to ask a question that had been troubling him: "How hard should I work, and at what?" Gittings was a shrewd judge of character. As Elliott remembered: "He strongly advised me to use my three years at Cambridge to enjoy myself in the interval before the next war"—advice that Elliott followed to the letter. He played cricket, punted, drove around Cambridge in a Hillman Minx, and attended and gave some very good parties. He read a lot of spy novels. On weekends he went shooting or to the races at Newmarket. Cambridge in the 1930s boiled with ideological conflict; Hitler had taken power in 1933; the Spanish civil war would erupt in the summer of 1936; extreme Right and extreme Left fought it out in university rooms and on the streets. But the fervid political atmosphere simply passed Elliott by. He was far too busy having fun. He seldom opened a book and emerged after three years with many friends and a third-class degree, a result he considered "a triumph over the examiners."

Nicholas Elliott left Cambridge with every social and educational advantage and absolutely no idea what he wanted to do. But beneath a complacent and conventional exterior and the "languid, upper-class manner" lay a more complex personality, an adventurer with a streak of subversion. Claude Elliott's Victorian rigidity had instilled in his son a deep aversion to rules. "I could never be a good soldier because I am insufficiently amenable to discipline," he reflected. When told to do something, he tended to "obey not the order which he had actually been given by a superior, but rather the order which that superior would have given if he had known what he was talking about." He was tough—the brutality of Durnford had seen to that—but also sensitive, bruised by a lonely childhood. Like many Englishmen, he concealed his shyness behind a defensive barrage of jokes. Another paternal legacy was the conviction that he was physically unattractive; Claude had once told him he was "plug ugly," and he grew up believing it. Certainly Elliott was not classically handsome, with his gangly frame, thin face, and thick-rimmed glasses, but he had poise, a barely concealed air of mischief, and a resolute cheerfulness that women were instantly drawn to. It took him many years to conclude that he "was no more or less odd to look at than a reasonable proportion of my fellow creatures." Alongside a natural conservatism he had inherited the family propensity for eccentricity. He was no snob. He could strike up a conversation with anyone from any walk of life. He did not believe in God or Marx or capitalism; he had faith in King, country, class, and club (White's Club, in his case, the gentleman's club in St James's). But above all he believed in friendship.

In the summer of 1938 Basil Fisher took a job in the City, while Elliott wondered idly what to do with himself. The old boys soon solved that. Elliott was playing in a cricket match at Eton that summer when, during the tea interval, he was approached by Sir Nevile Bland, a senior diplomat and family friend, who tactfully observed that Elliott's father was concerned by his son's "inability to get down to a solid job of work." (Sir Claude preferred to speak to his son through emissaries.) Sir Nevile explained that he had recently been appointed Britain's minister at The Hague, in the Netherlands. Would Nicholas like to accompany him as honorary attaché? Elliott said he would like that very much, despite having no idea what an honorary attaché might actually do. "There was no serious vetting procedure," Elliott later wrote. "Nevile simply told the Foreign Office that I was all right because he knew me and had been at Eton with my father."

Before leaving, Elliott underwent a code training course at the Foreign Office. His instructor was one Captain John King, a veteran cipher clerk who was also, as it happened, a Soviet spy. King had been passing Foreign Office telegrams to Moscow since 1934. Elliott's first tutor in secrecy was a double agent.

Elliott arrived at The Hague in his Hillman Minx in the middle of November 1938 and reported to the legation. After dinner, Sir Nevile offered him a warning—"in the diplomatic service it is a sackable offense to sleep with the wife of a colleague"—and some advice—"I suggest you should do as I do and not light your cigar until you have started your third glass of port." Elliott's duties were hardly onerous—a little light bag carrying for the minister, some coding and decoding in the wireless room, and attendance at formal dinners.

Elliott had been in the Netherlands only four months when he got his first taste of clandestine work and an "opportunity to see the German war machine at first hand." One evening, over dinner, he fell into conversation with a young naval officer named Glyn Hearson, the assistant naval attaché at the embassy in Berlin. Commander Hearson confided that he was on a special mission to spy on the port of Hamburg, where the Germans were believed to be developing midget submarines. After a few more glasses, Hearson asked Elliott if he would care to join him. Elliott thought this a splendid idea. Sir Nevile gave his approval.

Two days later, at three in the morning, Elliott and Hearson broke into Hamburg's port by climbing over the wall. "We discreetly poked our noses all over the place for about an hour" taking photographs, Elliott recalled, before "returning to safety and a stiff drink." Elliott had no diplomatic cover and no training, and Hearson had no authority to recruit him for the mission. Had they been caught, they might have been shot as spies; at the very least, the news that the son of the Eton headmaster had been caught snooping around a German naval dockyard in the middle of the night would have set off a diplomatic firestorm. It was, Elliott happily admitted, "a singularly foolhardy exploit." But it had been most enjoyable and highly successful. They drove on to Berlin in high spirits.

April 20, 1939, was Hitler's fiftieth birthday, a national holiday in Nazi Germany and the occasion for the largest military parade in the history of the Third Reich. Organized by propaganda minister Joseph Goebbels, the festivities marked a high point of the Hitler cult, a lavish display of synchronized sycophancy. A torchlight parade and cavalcade of fifty white limousines, led by the Fuhrer, was followed by a fantastic five-hour exhibition of military muscle involving fifty thousand German troops, hundreds of tanks, and 162 warplanes. The ambassadors of Britain, France, and the United States did not attend, having been withdrawn after Hitler's march on Czechoslovakia, but some twenty-three other countries sent representatives to wish Hitler a happy birthday. "The Fuhrer is feted like no other mortal has ever been," gushed Goebbels in his diary.

Elliott watched the celebrations, with a mixture of awe and horror, from a sixth-floor apartment in the Charlottenburger Chaussee belonging to General Noel Mason-MacFarlane, the British military attaché in Berlin. "Mason-Mac" was a whiskery old warhorse, a decorated veteran of the trenches and Mesopotamia. He could not hide his disgust. From the balcony of the apartment there was a clear view of Hitler on his saluting podium. The general remarked under his breath to Elliott that Hitler was well within rifle range: "I am tempted to take advantage of this," he muttered, adding that he could "pick the bastard off from here as easy as winking." Elliott "strongly urged him to take a pot shot." Mason-MacFarlane thought better of the idea, though he later made a formal request to be allowed to assassinate Hitler from his balcony. Sadly for the world, the offer was turned down.

Elliott returned to The Hague with two newly minted convictions: that Hitler must be stopped at all costs and that the best way of contributing to this end would be to become a spy. "My mind was easily made up." A day at Ascot, a glass of fizz with Sir Robert Vansittart, and a meeting with an important person in Whitehall did the rest. Elliott returned to The Hague still officially an honorary attaché but in reality, with Sir Nevile Bland's blessing, a new recruit to the Secret Intelligence Service, or MI6. Outwardly his diplomatic life continued as before; secretly he began his novitiate in the strange religion of British intelligence.

Sir Robert Vansittart, the Foreign Office mandarin who smoothed Elliott's way into MI6, ran what was, in effect, a private intelligence agency outside the official orbit of government but with close links to both MI6 and MI5, the Security Service. Vansittart was a fierce opponent of appeasement, convinced that Germany would start another war "just as soon as it feels strong enough." His network of spies gathered copious intelligence on Nazi intentions, with which he tried (and failed) to persuade Prime Minister Neville Chamberlain of the looming confrontation. One of his earliest and most colorful informants was Jona von Ustinov, a German journalist and fierce secret opponent of Nazism. Ustinov was universally known as "Klop," Russian slang for bedbug, a nickname that derived from his rotund appearance, of which he was, oddly, intensely proud. Ustinov's father was a Russian-born army officer; his mother was half Ethiopian and half Jewish; his son, born in 1921, was Peter Ustinov, the great comic actor and writer. Klop Ustinov had served in the German army during the First World War, winning an Iron Cross, before taking up a post with the German Press Agency in London. He lost his job in 1935 when the German authorities, suspicious of his exotically mixed heritage, demanded proof of his Aryanism. That same year he was recruited as a British agent, code-named "U35." Ustinov was fat and monocled, with a deceptively bumbling demeanor. He was "the best and most ingenious operator I had the honor to work with," declared Dick White, his case officer, who would go on to head both MI5 and MI6.

Excerpted from A Spy Among Friends by Ben Macintyre Copyright © 2014 by Ben Macintyre. Excerpted by permission of Crown, a division of Random House LLC. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.