The dread in Marvar’s eyes. Dyer had seen it again this morning when he threw a careless glance at the mirror. It pained him to look at the face that stared back. Marvar’s black spot had come into his hands, darker than anything that Blind Pew could have pressed into his palm, darker than anything he had a name for.

Except that it wasn’t nameless, it wasn’t torn from a bible or the Koran or from the book of any religion, and it wasn’t black; it was banana yellow, composed of quite specific numbers and letters, and it came tagged with a quote from a German chemist.

Did that fear in Marvar’s eyes explain his disappearance? Had the burden overwhelmed him?

Ludicrous as he had made Hissop’s suggestion sound, Dyer wondered if it was conceivable that Marvar had stuck to his original plan and taken Samir hiking, and now was somewhere safe. Hissop had read out a list of physicists who at regular intervals escaped into the mountains to keep sane – Teller, Fermi, Bloch, Heisenberg. Marvar’s abrupt departure caused Dyer to think of the still-missing Australian mathematician Todd Angle who had vanished while on a bush walk in Tasmania.

It made Dyer smile to imagine Marvar stumbling through rugged hill-country in mountain boots after he had buried his abstruse nuclear theory in the school sandpit. It was important to laugh. Otherwise it was all so damned unamusing.

Dyer climbed with Marvar a little way in his imagination. He wanted so much to believe that Marvar had arrived early at the Asselins’ to take his son off walking. He wanted to picture him rising through the grassy fells above Howtown with Samir, looking down to Patterdale below. Then he broke off a piece of rice cake and put it in his mouth. No, it was too hard to conjure a coatless Marvar struggling up Fusedale with his son, stumbling and wheezing. As for the alternative scenarios which Hissop had posited, in their violence they too closely resembled the games on Leandro’s Xbox. They lay beyond Dyer’s willingness to believe. Easier to imagine that Marvar had melted into Oxford’s thin air. He had disappeared in a storm, as Gustav Mahler was said to have done. He had burned himself out like a comet.

Yet if Marvar didn’t come back … if in some way he had been taken out of the equation … then that left Dyer sole custodian of his knowledge.

About what to do with this knowledge, he has no script. He’s not going to make a copy; Marvar wouldn’t have wanted that. ‘You don’t need any other.’ And the algorithm is too long for Dyer to memorise.

If he can’t copy or remember it, what other option does he have?