The most recent group had been caught and sent down, and after their expulsion had published a guide to the rooftops under the pseudonym Herodotus. Ever since its publication the university, acting for the safety of the student body and to protect its own reputation, had tasked its librarians with buying a copy whenever one arose for sale, and destroying it. Only a hundred or so had ever been printed. Even Francis’s budget was no match for the acquisitive resources of Cambridge, and whenever one of the books had come to auction, he had found himself outbid. There were now so few left in the world that the ones lingering in private collections were never likely to come back into circulation.

The original Night Climbers themselves, however, had been farsighted. In conversation with a sympathetic fellow, who had as a student joined the young men on their climbs but had stopped some time before they were caught, Francis learnt of the existence of a copy which was hidden in the very shelves of the Wren Library. The book had been torn into its constituent chapters, so that it might never be discovered and thrown away in its entirety. Each chapter detailed a different building, revealing all at once its secrets and pitfalls, together with the story of how it had been conquered. Francis was able to combine this knowledge with his study of the recent additions of motion sensors and CCTV cameras to predict with great certainty the course of our climbs. Much had changed in the forty-odd years since the book was written, but it was still an invaluable guide.

The key to finding each chapter lay in a copy of T. S. Eliot’s The Waste Land, which was positioned on the shelves in the place where the book itself ought to have stood, lingering amongst the works of the real Herodotus. The poem was high modernist, an arcane maze of literary references, some of which were underlined in pencil. Each marked reference, if properly understood, would lead the reader to a library shelf housing a chapter. So for example the first line, ‘April is the cruellest month’, which is taken by scholars to refer to the first line of Chaucer’s General Prologue, led Francis to The Canterbury Tales, where the chapter for St John’s College was tucked neatly against the wall behind the reference copy.

Francis always chose routes that allowed us to climb with no equipment. There were several reasons for this: we did not have the skills to employ grapples and crampons; and their use would have scarred the stone and damaged the tiles, something that was anathema to us. Most of all, however, it sullied the purity of the thing if we did not accomplish it with our own bodies, and our own ingenuity.

There was, however, one particular climb which could not be made without a rope. It was the ascent of the Senate House. The building had foiled the original Night Climbers, who had declared it impossible, and when Francis found no chapter describing it his curiosity was inflamed. It was a neo-classical building that made us all grateful for Cambridge’s Gothic heritage, with its excesses of carved stone. The straight perpendicular lines and stern proportions of the Senate House presented no handholds and no chimneys, just a barren face of rock. Rumour had it that if any man could get on to the roof they would be able to escape over the Senate House Leap. This legendary jump would take the climber over Senate House Lane and on to the rooftops of the college opposite, which furnished a paradise of eaves, ladders, arches and gargoyled gutters. Francis spent a long time studying the building, and finally devised a technique which would get one of us on to the roof unassisted. That person would then lower a rope for the others, and we would try the jump for ourselves.