Five minutes after leaving the car, Keller paused and with the barrel of his Glock made a straight-line gesture toward the ground. It meant they had reached the Ulster border. He turned to the north and led Gabriel across a series of pastures, each divided by hedgerows of blackthorn. The border was a few yards to their right. Once, there would have been watchtowers manned by Grenadier Guards and Hussars, but now only grain silos and barns marked the horizon. Keller, the bloodstained survivor of South Armagh’s dirtiest fighting, moved slowly, planting each step as though a mine were beneath his foot, breaching each hedgerow as if his killer waited on the other side.

After moving about a kilometer in this laborious manner, Keller led Gabriel across a rocky patch of ground between a pair of ponds. Before them rose a stand of trees, and beyond the trees was Jimmy Fagan’s farm in Northern Ireland. Keller crept forward, tree to tree, and then froze. About thirty feet away, shrouded in darkness, stood a man with an AK-47 at the ready. The gun was fitted with a carbon-fiber over-barrel suppressor, a serious weapon for a serious predator. Keller carefully removed his MI6 mobile and sent a pre-typed text message to Vauxhall Cross. Then he drew the knife from his pocket and he waited.

Because it was a domestic matter, Graham Seymour allowed Amanda Wallace to make the actual call. It arrived at the Crossmaglen base of the PSNI at 7:27 p.m., and within a minute several units were rolling into Newry Street, lights blazing. By seven thirty Jimmy Fagan’s phone was buzzing with text messages from his scouts.

“How many units?” asked Quinn.

“Six at least, including some tactical boys.”

“Where are they headed?”

“Down the Dundalk Road.”

“The wrong way,” said Quinn.

“Not even close.”

Another text hit Fagan’s phone.

“What does it say?”

“They’re turning right on Foxfield.”

“Still the wrong way.”

“What do you think it means?”

“It means you should tell your boys to be on their toes, Jimmy.”

“Why?”

Quinn smiled. “Because they’re here.”

At 7:31 the man standing thirty feet from Christopher Keller removed his right hand from the AK-47 and used it to remove a mobile phone from his pocket. The phone flared briefly, and in the glow of its screen Keller glimpsed the face of the man who would soon be dead. He was Keller’s age, Keller’s height and build. He might have been a farmer. He might have driven a lorry or done odd jobs. In another lifetime he had been Keller’s enemy. Now he was his enemy again.