A young man stuck his head around the door, looked first at me and then at the secretary – who was deep in an interminable phone conversation that seemed mostly about shakshouka, and how not to cook it. He went to speak and then thought better of it, smiling at me conspiratorially as he backed out into the corridor. A student, most likely. I guessed she had cultivated a reputation that kept all but the most persistent enquirers at bay.

I rolled my shoulders, sat up straight and looked around the office, at the file-stacked shelves and coffee-cup laden filing cabinet and at the sheets of names and dates and events pinned to the walls. A photograph of the Red Sea hung in a cheap gilt frame above the secretary’s desk; next to it a clock that I tried hard not to stare at. It didn’t look like it from where I was sitting, but the Kolymsky School of Computer Sciences was one of the best in the world – or so Baaz had told me. How or why Rachel had ended up there, I had no idea: in the years since I’d lost sight of her, I’d never once looked her up. Where she was, what she was doing, who she’d become – they were possibilities my imagination rarely got to grips with.

In fact, until recently, I hadn’t even thought of her that often. The memory of her – any memory of her – was so powerful, so visceral, that replaying the scenes we’d lived out gave me a physical jolt. More than one cup of coffee had been spilled when a stray thought of her tripped me up, unawares. But memory is an unreliable guide to the past. Ask any group of eyewitnesses to an event exactly what they saw and you’re likely to have as many different answers as people you interrogate. My mother had told me once that when memories are remembered, they are delicate, fragile, vulnerable to being rewritten.

‘Every recollection could be a fabrication, Max,’ I could hear my mother say, not long after my father had said goodbye, taking off on his latest trip to Africa. ‘Be careful you don’t just end up with fantasies.’

A fortnight later they were both dead.

I screwed up my eyes at the thought of the lake water and concentrated instead on what Baaz had found out online in a series of encrypted internet searches – which wasn’t much. The Kolymsky School of Computer Sciences was outstanding, but its supposed crowning glory – the Azriel Jacobs fellowship – barely featured online. Professor Rachel Levy was billed as a forty-four-year-old genius who, as Baaz explained with palpable excitement, had apparently spent her entire career wrestling with something called ‘NP-complete decision problems in computational complexity theory’.