By morning the scale of the calamity was plain to see. Most of the dead still lay where they had fallen—indeed, many still sat in their original theater seats. The commissioner of the Metropolitan Police declared the entire West End an active crime scene and urged both Londoners and tourists to avoid the area. The Underground canceled all service as a precautionary measure; businesses and public institutions remained closed throughout the day. The London Stock Exchange opened on time, but trading was suspended when share prices plummeted. The economic loss, like the loss of life, was catastrophic.

For reasons of security, Prime Minister Jonathan Lancaster waited until midday to tour the devastation. With his wife, Diana, at his side, he made his way on foot from the Garrick to the Prince Edward and finally to the St. Martin’s. Afterward, outside the Met’s makeshift command post in Leicester Square, he briefly addressed the media. Pale and visibly shaken, he vowed the perpetrators would be brought to justice. “The enemy is determined,” he declared, “but so are we.”

The enemy, however, remained curiously quiet. Yes, there were several celebratory postings on the usual extremist Web sites, but nothing of authority from ISIS central command. Finally, at 5:00 p.m. London time, a formal claim of responsibility appeared on one of the group’s many Twitter feeds, along with photographs of the fifteen operatives who had carried out the attack. A few terrorism analysts expressed surprise over the fact that the statement made no mention of anyone named Saladin. The savvier ones did not. Saladin, they said, was a master. And like many masters, he preferred to leave his work unsigned.

If the first day was characterized by solidarity and grief, the second was one of division and recriminations. In the House of Commons, several members of the opposition party lambasted the prime minister and his intelligence chiefs for failing to detect and disrupt the plot. Mainly, they asked how it was possible that the terrorists had managed to acquire combat assault rifles in a country with some of the most draconian gun control laws in the world. The head of the Met’s Counter Terrorism Command issued a statement defending his actions, as did Amanda Wallace, the director general of MI5. But Graham Seymour, the chief of the Secret Intelligence Service, otherwise known as MI6, chose to remain silent. Until recently, the British government had not even acknowledged the existence of MI6, and no minister in his right mind would have ever dreamed of mentioning the name of its chief in public. Seymour preferred the old ways to the new. He was a spy by nature and upbringing. And a spy never spoke for attribution when a poisonous leak to a friendly reporter would suffice.

Responsibility for protecting the British homeland from terrorist attack fell primarily to MI5, the Metropolitan Police, and the Joint Terrorism Analysis Center. Still, the Secret Intelligence Service had an important role to play in detecting plots abroad before they reached Britain’s vulnerable shores. Graham Seymour had warned the prime minister repeatedly that an ISIS attack on the United Kingdom was imminent, but his spies had failed to produce the hard, actionable intelligence necessary to prevent it. Consequently, he regarded the attack on London, with its horrendous loss of innocent life, as the greatest single failure of his long and distinguished career.

Seymour had been in his magnificent office atop Vauxhall Cross at the time of the attack—he had seen the flashes of the explosions from his window—and in the dark days that followed he rarely left it. His closest aides pleaded with him to get some sleep and, privately, fretted over his uncharacteristically worn appearance. Seymour curtly advised them their time would be better spent finding the vital intelligence that would prevent the next attack. What he wanted was a loose thread, a member of Saladin’s network who could be manipulated into doing his bidding. Not a senior figure; they were far too loyal. The man Graham Seymour was looking for would be a small player, a runner of errands, a carrier of other people’s bags. It was possible this man might not even know he was a member of a terror organization. It was even possible he had never heard the name Saladin.

Policemen, secret or otherwise, have certain advantages in times of crisis. They stage raids, they make arrests, they hold press conferences to reassure the public they are doing everything in their power to keep them safe. Spies, on the other hand, have no such recourse. By definition, they toil in secret, in back alleys and hotel rooms and safe houses and all the other godforsaken places where agents are persuaded or coerced into handing over vital information to a foreign power. Early in his career, Graham Seymour had carried out such work. Now he could only monitor the efforts of others from the gilded cage of his office. His worst fear was that another service would find the loose thread first, that he would once again be relegated to a supporting role. MI6 could not crack Saladin’s network alone; it would need the help of its friends in Western Europe, the Middle East, and across the pond in America. But if MI6 were able to unearth the right piece of intelligence in a timely fashion, Graham Seymour would be first among equals. In the modern world, that was the best a spymaster could hope for.

And so he remained in his office, day after day, night after night, and watched with no small amount of envy as the Met and MI5 rolled up remnants of Saladin’s network in Britain. MI6’s efforts, however, produced nothing of consequence. Indeed, Seymour learned more from his friends in Langley and Tel Aviv than he did from his own staff. Finally, a week to the day after the attack, he decided a night at home would do him good. The computer logs would show his Jaguar limousine departed the parking garage, coincidentally, at 8:20 p.m. precisely. But as he was heading across the Thames toward his home in Belgravia, his secure phone purred softly. He recognized the number, along with the female voice that came on the line a moment later. “I hope I’m not catching you at a bad time,” said Amanda Wallace, “but I have something that might be of interest. Why don’t you stop by my place for a drink? My treat.”