Richard Knox wasn’t a regular. In fact, he’d only been to the Gresham Arms once before. But sitting alone at a corner table, two empty pint glasses in front of him and another well on its way at one o’clock in the afternoon, he looked like he belonged. His tie was loose, the top button of his shirt undone. His suit jacket had been discarded on the stool next to him. His face was drawn, and his thick black hair, normally kept in place with a heavy pomade, had fallen over his forehead, concealing the old scar that stuck out from his hairline above his left temple. He looked like someone had recently done a number on him, because they had.

Two weeks ago Knox had been the darling of MI5. The officer who had broken the Calder Hall Spy Ring, a network of Soviet sympathisers who had been passing classified information about the nuclear plant at Sellafield in Cumbria to the KGB. Two hours ago he had been summarily suspended from the Service, escorted from its headquarters in Leconfield House in Mayfair, and found himself walking the short distance to the Gresham Arms. It was a sunny morning, but he’d felt very much out in the cold.

The breaking of the Calder Hall network—made up of Nigel Slaughter, a security guard at the nuclear plant, Patrick Montcalm, a travelling salesman, and Sandra and Peter Horne, a retired couple in Richmond with a house full of radio equipment and one-time code pads—had been a major triumph for Knox.

Rumours had lingered about Soviet infiltration of MI6 ever since Guy Burgess and Donald Maclean had disappeared from a trip on the cross-Channel ferry SS Falaise and popped up in Moscow a decade ago. Then those same quiet voices had started talking about moles in MI5. It was Knox’s job to find any leaks and, if they existed, stop them.

Both Knox and James Holland, the director general of MI5, were convinced that groups like Calder Hall couldn’t flourish without someone inside the intelligence community supporting them, and that breaking the network would be a crucial step in discovering whoever this invisible someone was. But three days ago, on the night of Sunday 9 July, Holland had been found unconscious and unresponsive on the floor of the living room in his Highgate home by his wife, Sarah. His personal diary showed that he was supposed to be having dinner with Knox, his loyal deputy, but Knox hadn’t appeared.