'You're late,' she whispered as she handed him his staff of office – a rainbow-patterned umbrella.

'Je mi lito,' he replied, apologizing being one of the few aspects of Czech he had mastered. 'I overslept.'

Ivana's smile stiffened only slightly as she set about introducing him to his charges. A doctor of history, she called him, in order to forestall any complaints about his clearly not having been born and bred in Prague. It was not a technically valid description. Umber had never finished his doctorate. But, in another sense, which afforded him some wry amusement, it was true. There would be a little doctoring of history before the tour was over. He could guarantee that.

There was one latecomer who settled up with Ivana after she had said her preliminary piece. Having failed to register the man's presence on the tram. Umber naturally made nothing of his last-minute arrival. Ivana wished them a good day and bustled off to the bank with the takings. She would soon be back in the warmth and relative comfort of the Jolly Brolly office. A phone call to Janousek, proprietor of U Modre Merunky, where they were scheduled to stop for a 'typical delicious Czech lunch', and her duties would be concluded.

Lucky her, thought Umber, as he took a deep breath of cold Czech air and launched his commentary with some loosely framed thoughts on the Prague Spring of 1968 and the Velvet Revolution of 1989. It was a well-worn theme. He was a historian, after all, albeit not as well qualified as Ivana had implied. He was on autopilot before they had even reached the Monument to the Victims of Communism.

And on autopilot he remained as the tour proceeded. They reached Old Town Square in good time to see the Astronomical Clock's march-past of apostles when it struck the hour, crossed Charles Bridge, popped into and out of the Church of St Nicholas, then took the funicular railway (fare included in the price) up to Petrin Park. The snow was ankle-deep in the park, which slowed their progress, those inadequately clad and shod only now realizing what they had signed up for. Umber had allowed for this, however. Some deft abridgements during their visits to the Strahov Monastery and the Loreto had them at U Modre Merunky, halfway back down the hill to Prague Castle, more or less when Janousek was expecting them.

The exact nature of the deal between Ivana and this less than glorious example of Czech innkeeping had never been disclosed to Umber. It was certainly not predicated on the quality of the food. The roast pork was gristly, the red cabbage vinegary and the dumplings unyielding. But no-one complained. Those at Umber's table even praised the food. Perhaps they did not wish to hurt their host's feelings. Umber could have told them, but did not, that Janousek did not actually have any feelings on the subject that could be hurt.

The latecomer to the tour, Umber's fellow-passenger from the number 24 tram, sat at a different table and said little to his companions. Removal of his woolly-hat revealed a bald head with a dusting of shaven white hair above his deeply lined brow, piercing blue eyes and hollow cheeks. He was a short, broad, bony man of sixty or seventy, whom nobody seemed eager to engage in small talk and who looked as if that suited him fine: he was nobody's fool, his bearing proclaimed, and nobody's favourite uncle either. His gaze appeared to be fixed throughout the meal on the back of David Umber's head. But of that David Umber was unaware.

Lunch over, but already repeating on some of them, the group slithered down to the Castle in time for the two o'clock Changing of the Guard. This was followed by a circuit of St Vitus's Cathedral before they made their way to the Royal Palace for Umber's account of the famous Defenestration of 1618 that sparked off the Thirty Years War. He was mildly worried at this stage by the chance that someone might ask him to explain the whys and wherefores of that long-ago conflict. But the moment passed with questionless ease. They made a gingerly descent of Old Castle Steps, crossed back over the river and entered the Jewish Quarter.

Three synagogues and one cemetery later, they returned to Old Town Square, where the tour ended at the birthplace of Franz Kafka. Umber cracked his customary joke about hoping nobody had found the day too much of a trial. There were more smiles than laughs and a few expressions of thanks, extending in one case to a (very) modest tip. Then the group dispersed.

It was late afternoon now and growing colder. Umber hurried round to the Jolly Brolly nerve centre, two second-floor rooms about halfway between Old Town Square and the Prague branch of Tesco, where he planned to buy his dinner.

There was no sign of Ivana in the office. She had left it in the languid care of Marek, her youthful and, in Umber's view, useless assistant. Marek was sitting with his feet on the desk, smoking a Camel cigarette and texting a friend when Umber walked in. Marek nodded a greeting and slid a small, square manilla envelope across the desk. Umber pocketed the envelope, returned his umbrella to the stack in the corner and made to leave.

At which point he noticed that morning's edition of Annonce - the classifieds paper with the most comprehensive accommodation listings in Prague – lying discarded in the waste bin. He fished it out and glanced enquiringly at Marek.

'Prosim,' said Marek, with a sarcastic smirk.

Umber exited, checking the contents of the envelope as he descended the rickety stairs. All the money was there. But all, in this case, was not a lot.

Back on the street, solvent if scarcely flush, Umber decided that Tesco could wait. Jolly Brolly HQ's proximity to U Zlateho Tygra, the Old Town's most famous drinking establishment, verged on the unreasonable. At this hour he could be sure of a seat, which, after foot-slogging round the city all day, he needed almost as much as a beer.

U Zlateho Tygra – the Golden Tiger – was its normal soothing, smoky self. Umber settled himself at the table screened by the pub's trophy cabinet, next to the window on which the eponymous tiger frolicked in stained-glass abandon. Half a litre of cellar-cooled Pilsner was swiftly delivered to him and his tab initiated with a slash of the server's pen. Umber took a deep gulp of beer, then unfolded Annonce and commenced a less than hopeful search for attractive and affordable alternatives to his present abode.

But his search never even reached the apartments to rent page. A bulky figure rounded the trophy cabinet at that moment and assumed a looming stance above him. Umber looked up and, to his surprise, recognized the newcomer, or at any rate recognized his outfit of maroon parka and matching woolly-hat. He was one of the tour party.

'Hello,' said Umber. 'What brings you here?'

'You do.' The man pulled off his hat and unwound his scarf, fixing Umber with a steely blue gaze.

It might have been the quality of the gaze that clinched it. Or it might have been the flat, faintly menacing tone of voice. Either way, recognition – true recognition – dawned now on Umber.

'I don't believe it,' he murmured. Which was true. He did not believe it.

'You'll have to,' the other man said. Which was also true. It was not a matter of choice. It never had been.

It had begun at Avebury. But it had not ended there.