‘He’s not here.’ She tried to block Knox’s view of the office, but he could see straight over her head. The room was untidy. Open books had been left on every surface, and several trays containing the remnants of old meals were strewn across the floor. But the woman, who Knox assumed must be Kaspar’s assistant, was right—he wasn’t inside.

‘Do you by any chance know where he might be?’ Knox asked.

The woman checked her watch. ‘It’s twelve thirty. He’ll be on Sheep’s Green.’

‘And how might I recognise him?’

‘He’ll be the one with the swans,’ she replied. Then she slammed the door.

Knox decided there was no point trying to get any more information from her, so he traced his way back through the labyrinth of the Cavendish and made his way the short distance to Sheep’s Green.

The warm morning was turning into a hot afternoon. The River Cam was clogged with punts, and day-trippers swarmed over the small plot of open land where the river split in two. It was one of the prettier patches in the city that wasn’t locked behind a college’s walls.

Knox could see several people trying to tempt the swans on the river with bits of food, but only one of them was having any success. An old man, leaning on a stick, was doling out crusts of bread to the birds. He also looked like he was deep in conversation with several of them. Knox wondered if this whole trip might have been a trick by. White.

He kept a short distance between himself and Kaspar. Knox didn’t want to get too close to the swans. He didn’t like swans, and these ones looked like they didn’t like anyone but Kaspar. Everyone else who approached them received either a hiss or a wide flapping of wings.

‘Are you just going to stand there?’ the old German said.

It took Knox a moment to realise Kaspar wasn’t talking to one of the swans but to him. ‘You think after all this time I don’t know when a spook is sneaking up on me?’ he asked.

Knox took this as his cue. He sidled up next to Kaspar, facing the river and doing his best to ignore the angry noises coming from the nearest swan.

‘I’m sorry for the intrusion,’ Knox said. ‘But I need your help.’

‘I’ve already told your people everything I know. I’ve told everyone everything I know’ He broke another crust into pieces and threw them at the swans.

‘This is something new. Something, well, something that we don’t know what it is.’ Knox pulled the bundle of equations half out of his jacket pocket so Kaspar could see them. ‘We need an expert opinion.’