She was a tourist, but he knew the system. He ran across the tracks and caught a fast train to Calais just as it was leaving, knowing she would miss it because the boy slowed her down. He arrived forty minutes before her train pulled in. As they alighted onto the platform and set off to purchase tickets for the P &O boat, he followed at a discreet pace, ready to intervene if she decided to find a police officer.

“It’s too cold to be out here,” Madeline complained, gripping her son’s hand tightly as he stood at the rail of the ferry. “Let’s go back inside.”

“Why did you leave my Spider-Man bag behind?” asked Ryan. His scarf and her warmest jacket had been folded away in it, but the bag had been left in their room during the rush to leave the hotel. He looked back out at the ice-grey channel and pointed to the sickly amber mist forming close to the water. Snow had begun to fall in thick flakes that stuck to his eyelids.

A steward tacked his way towards them. “Can you go back inside, please? The deck’s too slippery to be walking on. We’ll be docking in twenty minutes.”

As she pushed Ryan towards the doors, Madeline glanced back at the sea and wondered if there was time to throw the incriminating envelope overboard. She no longer wanted to take it to the police; it tainted her, pulling them both back, a harmful omen that reminded her of the mariner’s albatross.

She wanted to be home in Waterloo Road, where even her husband posed a smaller threat than the disturbing stranger who had invaded their life in France. She tried to imagine any circumstances that would present her discovery in a different light, but knew the truth in her heart; that he had killed and robbed and gone undiscovered, and would do so again if he felt threatened. She had learned to recognise the poisons that could fester and ripen inside him, knew it was her duty to warn the authorities, but feared they would bully and perhaps even implicate her. She could not find the energy within herself to set the process in motion. Instead, she was taking the coward’s way out and running away. There would be no more confrontations with violent men. She had to think of her son’s safety.

As the ferry lowered its great steel doors on the snowswept dock, she waited with her hands on Ryan’s shoulders, preventing the impatient boy from charging forward.

“Are we going to catch another train?” he asked, looking up at her. “Can’t we get a car?”

Suddenly driving seemed a better option; she would be able to hire a vehicle and take Ryan to the Southwest. She had relatives there, and it would be a way of making up for her lack of judgement with Johann, to let him enjoy some of the wonderful places she had never been able to see as a child. They could drive back to London before the money ran out. “All right,” she told him. “We’ll visit your aunt in Cornwall. We’ll hire a car.”

At customs, her fingers closed around the packet containing Johann’s other identity. Its secrets were burning her hand. She wanted to speak out, but the sour-faced young officer who checked her passport and waved her through showed no inclination to even acknowledge her presence.

As she made her way to the EasyCar kiosk, she had the sensation that she was being watched. Most likely there were CCTV cameras trained on them, checking for aberrant behaviour patterns and warning signs among the new arrivals. Surely he would never come here, where so much public life was monitored by security systems? Yet he had seemed entirely comfortable in Monaco, the most heavily policed country in the world. He was so convinced that no-one would ever be able to catch him that he had tested himself there.

She recalled the way he kept looking for the cameras in each street they entered, almost daring them to pick him out. How close had she come to placing herself and Ryan in danger? His victims were chosen for the sake of expedience, to gain their identities. This fact alone made him mystifyingly complex; he was no serial killer, attacking for gratification. Instead, he seemed to view his actions as the mere removal of obstacles standing in his way. The pattern, she had learned, was classic.

“Mum, she’s talking to you.” Ryan tugged at her arm, pointing to the car hire lady.

“Did you want a manual saloon or an automatic?” asked the counter girl.

“Automatic. I need to drop it off in London.” While she filled in the forms, Ryan wandered to the glass wall and looked out at the falling snow. He was making patterns in the condensation when he saw Johann walking across the slush-scabbed forecourt towards the truck park. Opening the door, he slipped outside.

“Johann!” he called, running after the man he had started to consider his new father. “Wait, we’re over here!”

Johann stopped in mid-stride and looked back. When he recognised the boy, he waved back unsmilingly.

“Can you come with us?”

“I’ll be with you soon, Ryan, I promise.”

“Mum’s taking me to Cornwall. She’s in there hiring a car. Let me get her.”

“No, don’t do that.”

“But you don’t know where we’ll be.” He hung on to Johann’s arm.

“Don’t worry, Ryan, I’ll find you.”

Madeline was coming out of the car hire kiosk, studying her receipt as she walked. He caught up with her in the snowy shadows of the dockside, the treacherous swell of ice-grey waves rising and plunging beside them.

“I don’t understand you,” he said, seizing her arms, holding her close. “You run away from me before I can explain, so I have to come after you. I know I am bad, I know what I have done, but you can save me, Madeline, you can make me good.”

“Leave me alone.” She was forced to shout because the wind was so strong in their ears. “You’re a murderer.” There were other words, but they were lost to the whirling sky.

“Yes, it is true, I cannot deny what I have done. But you-‘

As she crushed Ryan to her side and ran from the quay, slipping on sea-wet concrete, she thought, He means to kill us both. I’ll never let him near Ryan, never. Whatever I do, no matter how terrible, it will be for the sake of my son.

DS Janice Longbright closed her mobile and perched on the edge of the desk, crossing her legs in a slither of caramel nylon. It was now 5:45 P.M., and Giles Kershaw had returned with his preliminary notes on the examination of Oswald Finch’s body. The shell-shocked members of the PCU had been gathered in Longbright’s office, although no-one had yet managed to contact Raymond Land, who had last been seen tottering back from an extended Masonic luncheon with his Home Office liaison man, Leslie Faraday, in Covent Garden.

“Have you spoken to John and Mr. Bryant?” asked Dan Banbury, following the unit’s odd tradition of referring to May by his first name and Arthur by his last.

“It’s not necessary to raise your hand, Dan, you’re not in school. No, I thought I’d call them in a minute, with all of us here. Giles, have you got a time line for us?”

“Hang on a mo.” Kershaw unfolded his spindly legs and rose to the blackboard he had erected under the window. “I’d usually PowerPoint my notes to you all, but we have no network.” He glanced accusingly at Banbury, who seemed not to mind. Bimsley had chosen to sit next to Mangeshkar, who had moved her legs as far as possible away from him. April sat at the back, watching intently, her arms folded protectively across her chest.

“The bruise on Oswald’s neck wasn’t the only one,” Kershaw explained. “I found another, identical in shape and discolouring, on the left side of his chest. It would appear he suffered a thrombotic attack after getting thumped on the opening of his pulmonary artery and aortic valve, which prevents blood from reversing its flow back into the left ventricle of the heart. The convulsion interrupted the rhythm of his heart and stopped it. The whole thing happened very quickly, and was over in a few seconds. He was standing when this happened, and immediately fell down beneath the counter.”