COLONEL ARKADY PETROV was at his family dacha just west of Moscow when he took the call. Like so many Muscovites he liked to be out of the capital at weekends, get back to nature in this pretty wooded village where Boris Pasternak was said to have written Dr Zhivago. When Coronavirus had rampaged through Moscow, carrying off so many of his countrymen and -women, this was where he had sat it out with his family, a loaded hunting rifle always at the ready in case any unwanted and infected intruders should venture too close.

Now his phone was ringing in his pocket just as he was turning over the skewers of shashlyk on the patio grill, the scent of aromatic spices coiling up from the kebabs into the chill evening air. Colonel Petrov did not like to be disturbed on a weekend. He gave the skewers a final turn, wiped his hands on his apron and took the call, his brow furrowing in concentration as he listened to the voice of his subordinate at the other end. It was a long call and the news was promising. Their investigations had turned up a suspect, who was in their custody now and was currently having ‘an introductory conversation’ with his interrogators downstairs. Would the colonel like to come and take over the questioning in person? He looked at his watch. The timing was inconvenient, but he knew he must do this. He would have to drive all the way back into Moscow and take charge. Colonel Petrov ended the call, cast a brief, longing look at the half-cooked shashlyki and tipped them into the bin.

Forty-five minutes later he pulled up at the address they had given him. It was in Cheryomushki district, in the south of the city, nowhere near GRU headquarters. His men were not stupid: this whole investigation needed to be off the books and out of sight. Officially it did not even exist. The building looked like any one of a hundred other uniform tenement blocks in this former dormitory of the Soviet Union’s military-scientific community: tall, faceless, somewhat dilapidated. Colonel Petrov stepped over the small mound of wind-blown litter that had piled up beside the concrete entrance awning and nodded to the man on the other side of the glass door: one of his own. The door opened. Petrov stepped inside and followed the man down the steps to the laundry room. A well-built man in jeans, white T-shirt and black anorak stood guard at the top of the stairs, making sure no one followed: another of his own.