Walking down to College the next morning, they could have been in almost any town where the homes were primarily narrow three-storey Victorian townhouses of grey or red brick, with white-painted eaves and shallow front gardens: a low wall, an overgrown shrub, a paved path to a tiny porch with a gloss-painted front door. A little triangle of park provided a landmark as they turned down another street and suddenly Parker’s Piece opened before them.

House-hunting with his father had only involved driving the streets immediately around the station rather than exploring further, while Nick’s trip to Cambridge for his admissions interview (the Replacement was drafted in to accompany him when Michael got caught up with a work ‘emergency’) had taken in the centre of town and little more. Nick had yet to walk the route from the new house to College, so though he’d spent the last few months studying maps of Cambridge and Google StreetView, it was bewildering trying to make it match up with reality. Parker’s Piece was a little green rectangle on the map, a blurry stretch of browny-green on StreetView, not an immense expanse of torn-up grass that dwarfed the surrounding buildings – a school, the police station – turning them squat and ungainly.

They passed two football games going on side by side, then cut across on the diagonal path, past an ornate green and red lamp post with ‘Reality Checkpoint’ scribed into the paint: the Cambridge version of entering Narnia. It might as well have read ‘This is where it all gets weird. From here on, it’s another world.’

‘That’s Downing College,’ said Michael as they emerged on to Regent Street opposite the gates. ‘And ahead is Emmanuel – Emma to you, now you’re a student.’

They passed from plastic shop-window displays to the long grey frontage of Christ’s College on the right, all crenellations and regimented windows stretching to a huge carved-stone doorway. Michael led them to the left, into the pedestrian zone, turning down what Nick remembered from his maps as Petty Cury. A series of ugly 1960s concrete blocks, with shops in the bottom floors and offices above, gave way to the market: a cramped square of tiny temporary stalls, all pipework and striped awnings, with Great St Mary’s Church towering above. In a way it was ordinary enough, but not quite: like the world was slowly starting to change shape, strangeness drifting in, gradually taking over.

‘Residency rules are that students have to live within three miles of GSM during term-time,’ Michael said, gesturing at the church tower. ‘For three miles in every direction you’re officially within the “precincts” of the University. Odd, really, since Senate House is just there.’ He pointed past the tall spiked railings around the churchyard to the building opposite. ‘Senate House is where you’ll come for your exam results. And Graduation, of course.’

Nick had seen it before, but couldn’t help thinking it uninspiring next to the grandeur of King’s College. It was too white, too stark: all clean straight lines, like a Regency stately home. Even the colonnade was disappointing, too few arches, too much heavy white stone between them. Squatting next to the soaring majesty of King’s Chapel, Senate House’s too-green, too-stripy lawn seemed staid and ordinary. A stone plinth in the middle of the grass bore a hideous urn, the copper stained turquoise with verdigris. Maybe in a different light King’s would look smudged and dirty, but in the sunshine it glowed cream and gold, all gorgeous stone tracery and tiny jutting spires, too beautiful and delicate to be real.

Here it was: the Cambridge of photos. The Cambridge that didn’t seem part of the real world, as if time existed differently here. As if the past was the present, overlapping, interlocking, not one or the other but both.

Nick tried to keep his gaze roving, taking in everything, but it kept returning to the folly of the King’s frontage: a long thin wall pierced at intervals with open lancet windows and topped with a stone trellis, complete with purposeless little spires. Everything about it was pure excess. An extravagance of beauty.

I can’t imagine ever being truly miserable here.

‘Why would you be miserable at all?’ Michael asked.

Nick started, not used to his father paying enough attention to hear him when he mumbled. He was still trying to think what to say when they plunged into the gloom of narrow, cobbled Senate House Passage and the moment was gone.

‘So we’ve got Gonville and Caius on our right, but that’s pronounced “keys”,’ said Michael. ‘Oh, and remember Magdalene is “maudlin”. That’s Old Schools, behind Senate House on the left, and Clare College opposite. The gate at the end of the path goes to King’s Chapel, but it isn’t open all day: mostly you can count on it around Evensong, which you’re entitled to go to whenever you want. And here we are: Trinity Hall.’ They stepped into the Porter’s Lodge, Michael nodding to the porters at the desk as he led the way past the fireplace. ‘Now, all these little shelves are called pigeonholes.’

‘For post,’ Nick said, moving to his father’s side to peer at the names. His pulse kicked oddly as he looked for his own.

‘Remember to check every time you come through,’ Michael said. ‘Here we go.’

Derran, N.

Nick found himself smiling, reaching out to touch the name tag as if it were something extraordinary. ‘Is there always someone on duty in the pee-lodge?’

His father laughed. ‘It’s pronounced “plodge”. And, yes, any time you’re likely to be here there’ll be someone on duty. Just remember that most of the porters are ex-military at TitHall and act accordingly.’

Nick couldn’t help the snort.

Michael rolled his eyes. ‘There’s no point sniggering every time someone says it.’

‘They seriously couldn’t think of something better to shorten it to? Like T-Hall? Or you could say you were “in THrall”. It’s a cheat but it would be funnier.’

Michael sank on to one of the wooden window seats looking out into Front Court and took off his shoe, tipped a shard of pebble out of it. ‘You have to remember it’s pretty recently that the University opened up to more than a token number of students not from public schools, let alone girls, so you get what you’d expect from a language invented by Etonians.’

‘No wonder the locals mock the students.’

Michael pushed himself back to his feet. ‘Townies, Nick. Students are gownies and locals are townies.’

‘I know. There was a “basic Cambridge vocab” list in my Induction pack.’

‘That’s practically cheating. You’re meant to spend at least the first term never quite sure what anyone’s saying.’ Michael turned from the window with a grin only to start when he realised that Nick was standing at his shoulder, beaming up at him expectantly.

Nick’s smile faded with his father’s. ‘What’s wrong?’ His eyes darted away from his father’s sudden worried frown. ‘Did you remember something about work?’

Michael coughed, dug his hands into his pockets. ‘No, no. Just thinking I’m … Well, I’m really proud to be here with you today. Pretty cool to be introducing my fifteen-year-old to College. Wonder if I’ll bump into anyone I know.’

Nick moved his face into what should have been a smile, but somehow wasn’t.

Michael ducked his head and pushed through the wood and glass p’lodge door into Front Court, hurrying down the central flagstone path while Nick dawdled behind, staring up at the stone buildings all around. To the left and right were two storeys of tall windows under grey slate roofs set with a third storey of matched garret windows. The stone blocks of the walls, a strange creamy golden-brown, seemed almost to glow. To the left and right, the courtyard buildings were broken on the ground floor by an arch: on the left side was the chapel, marked out by a pair of two-storey stained-glass arch windows, a third smaller one perched up in the far corner. On the far side of the courtyard, opposite the main gate, was a double set of dark wood doors; above, the walls rose up to a triangular apex, decorated by moulded scrollwork and a crest below a grey-painted hexagonal plinth bearing a little cupola: thin white pillars supported a tiny silver-blue dome surmounted by a finial spike emerging through a golden ball.

The courtyard itself was quartered into neat little squares of lawn by a flagstone path edged in cobbles. Along the walls, narrow flowerbeds and window-boxes spilled over with geranium and lobelia and wallflowers. The girl in tiny pink hotpants standing in the centre of the courtyard, tapping a message into her mobile, was jarringly alien.

Nick had memorised the map of the College, but like everything else in Cambridge it was far more higgledy-piggledy than any plan allowed for. Michael cut right in the centre of the courtyard and through the low, narrow arched tunnel into North Court, all brown brick and black bike racks. He led the way up the steps to the left, past the bar, to the JCR.

‘Junior Combination Room,’ he reminded Nick, ‘not Common Room.’

Nick gave a hum of assent as he stepped up to the huge plate-glass windows that fronted the tattered room, turning his back on the stained carpet and filthy cushions upholstered in vomit-coloured fabrics.

‘Mumbling again?’ his father asked. ‘You’ll have to learn to speak up in supervisions, you know.’

In the glass, his face...