The moment Pam's back was turned, he slipped from the safety of the store entrance and ran off down the street. He hated to do it to her, but by the time she missed him he wanted to have vanished from her sight.

ARTHUR BRYANT halted in the middle of Battersea Bridge and gripped the handle of his umbrella tightly as the wind tried to snatch it away. He looked along the line of the river, but his vision was obscured by squalls of gale-driven rain and sleet. The city was well-protected from the elements, but here above the Thames, away from the sheltering mansion blocks, a man could catch pneumonia. Why Harold Masters had persuaded him to move their monthly meetings a week forward wasn't entirely clear. The doctor had mumbled something about 'being on hand' to help a friend in trouble, and had promised to call the others, so Bryant had set off to visit his friend unmindful of the late hour. Which was fine by him, because after all, they were jokingly known as the Insomnia Squad.

The elderly detective was not enjoying his sabbatical one bit. His partner John May was busy making a fool of himself over a woman in a closed-for-the-season hotel somewhere in the Greek islands, and would not be back until the new year arrived or his passion departed. The damage caused by the fire that had virtually destroyed their offices six weeks earlier was supposed to have been fully repaired by now, but thanks to a remarkable number of malingering carpenters and decorators claiming that they couldn't get the parts, their vans had broken down in Wandsworth, their wives had left them and so on, the work had yet to be finished. May had swanned off leaving his partner in an office without a roof, and so far he had not even sent a postcard detailing his absurd affair with the former Miss Ghana, or somewhere like that, who was young enough to be his niece. It really wasn't good enough. May knew what he was like when he couldn't work, how wound up he became. Not even a phone call.

So when Harold Masters had called twenty minutes ago, Bryant had jumped at the chance to get out of his flat and start solving something. 'I think you'll be intrigued by this,' the doctor had told him, whetting his appetite. What could he have meant? Still, it would help to find a cab right now. What did they all do in the rain, melt away?

Damn and blast, thought Bryant, setting off with renewed vigour, I must be half-mad to come out in weather like this. And of course, there were many in the metropolitan force who believed he was.

There was someone walking behind her, she was sure of it. There had been for the past five minutes or so. Pam quickened her pace, cutting down into Bond Street and checking the reflections from the street in each window she passed. There were three people following her route, a woman laden with Christmas shopping bags, a young Asian man in a track suit, an older man in a long black raincoat. None of them seemed to be paying her special attention. Releasing a cautious breath of relief, Pam doubled back to the tube station. She was furious with Vince for running off and leaving her, but she had decided to go to Red Lion Square and keep a watchful eye on him anyway. Display her initiative. Show some leadership qualities. Besides, she was curious to see if any members of this mythical League would put in an appearance.

Vince studied the buildings as he passed, endless terraces of Victorian homes, carved into tiers of offices. It was close to midnight now, but even at this late hour there were a few lights in the windows, legal and medical secretaries still at their desks, chatting on phones, talking to colleagues. These days a lot of people felt that they could only get their work into some kind of manageable shape by waiting until their switchboards had closed for the night. Pleasant, ordinary people, seated in cream-coloured rooms with great closed fireplaces, toiling quietly, sipping coffee, as unapproachable as if he was viewing them from a telescope a thousand miles away.

He walked through Queen Square, where a few doctors, nurses and students would still pass each other through the night on their way to Great Ormond Street Hospital, and the great dark weight of London surrounded him. Layers of history compressed like geological formations were here beneath his feet, ancient countryside lying in disguise. These roads were older than history, their paths twisting to preserve routes around long-dried marshes, their walls following the borderlines of land once defined by hedgerows. A two-thousand-year-old city that had stood in darkness for all but the last century. So many murderous deeds had occurred here beneath the heavy cloak of night, and yet he felt protected, as though the metropolis would ultimately prove itself to be on his side.

The air was sharper here than in the traffic-choked West End. Red Lion Square stood dark and empty before him, the bare branches of its trees entwined in symbiotic dependency. The new buildings stuck out among the old, square and plain, of lighter brick. They marked the sites of the Blitz's bombs more clearly than if the land had been left razed.

He was not scared now. Melancholy and a little tired, hungry even, but no longer frightened by the prospect of what the night still held in store, even though – incredibly, it seemed – there were still seven full hours of darkness ahead. It felt strange to be so completely alone, even though there were friends and family out there in the dark. And Pam – what had she said about the ghosts walking diagonally across the little park?

He reached the main entrance to the square and found the gate padlocked, but it was an easy matter to climb over the low railing and follow the path inwards. He moved beyond the reach of street light to a point where he could barely see the way ahead, but could already make out the sharp white rectangle of the envelope propped up in the circular central flowerbed, waiting for him.

Pam briskly cut into the backstreets beyond Holborn tube station. She remembered most of the roads around here from her temping days. Having been forced to wait nearly fifteen minutes for a train, she felt sure that she would arrive too late to find Vince.

She entered the southern side of Red Lion Square, keeping against the walls wherever possible, and watched. There was someone in the little park; she could see a bluish shape moving between the bushes. From her vantage point it was difficult to identify the figure, but just at that moment it turned, and her friend stood revealed in profile. He stooped, disappearing for a moment. Then reappeared, holding something in his hands and studying it.

There was someone else in the park with him, the dark shape of a man standing motionless behind a hollybush. Marooned where she was, Pam could do nothing but watch and wait. Presently Vince left the park, vaulting over the railings and setting off at speed. The remaining figure shifted off through the undergrowth, making for the top side of the square. It was impossible to tell if this was one of Vince's phantom tormentors or simply a loony locked out of his shelter.

She slipped from the protection of the office doorway, keeping him in her sights. He was a hundred yards from her, then eighty, then fifty. He was wrapped in a heavy, expensive-looking overcoat, murmuring into a mobile phone.

She needed to be nearer still, but the pavement between them was broad and empty, too brightly lit, and her shoes made too much noise. Vince had gone, and for all she knew she could be stalking some poor innocent businessman. But he had been in the park, hidden and watchful. It was worth the risk. As he shut the phone and moved off, Pam moved with him.