‘A revolutionary breakthrough,’ echoed Hissop, taking another bite.

Dyer looked from Hissop to Updark, who would have to be calmed down and outwitted. ‘I’m sorry, but what are you talking about?’

‘You know, John, I have a little problem with stupidity above a certain temperature.’

‘You’ve just praised my intelligence.’

Updark looked at him severely. ‘I don’t think we’re hearing each other.’

Dyer only smiled. ‘I’m all ears, Lionel.’

‘Let’s start with: I am going to take everything you’ve told me so far cum grano salis.’

‘But I haven’t told you anything. Plus, I’m not sure what I’ve got to say. And if I did have something to say, you have reminded me I am under no obligation to say it. Actually, I will have a flapjack.’

The mask of affability had definitely slipped by the time Updark resumed.

He said to Dyer – it was important this was made clear, ‘This is D-notice, right? Right, John? National security. You tell no one. But no one.’

‘Right.’

‘There is a possibility – I stress possibility – that Marvar has bypassed the whole bunny game and found a new way to solve our energy problems.’

Dyer tried to consider this calmly. ‘What makes you think that?’

‘You’d hardly expect me to reveal sensitive information. Let’s say that data has been discovered on one of the machines at the lab, and our cuckoos in the Clarendon seem to think it points to a result. Just how Marvar achieved this result is the mystery that is confronting us.’

‘It sounds,’ Dyer said, ‘as though you were already monitoring him.’

‘We’ve found his computer,’ said Hissop, sidestepping. ‘We’ve found a safe he used. We’ve searched his rooms. He’s been crafty. He’s left no trace.’

‘Apart from this,’ said Updark, and flicked his fingers. ‘Give us that napkin, Lorna, will you?’

She opened a drawer in the cabinet and brought something out and put it down next to the biscuit tin.

Updark said: ‘We picked this up in the café where he liked to take breakfast. The waiter overheard Marvar talking to himself before he leaped up and rushed off. He sounded pretty excited about something – so excited that he forgot to ask for the bill. This was last Friday morning. One week and three days ago.’

He unfolded the white paper napkin with tremendous care, as if it might detonate. Scribbled on it in black ink, among the coffee stains and dried egg yolk, was the beginning of what looked like a mathematical formula.

‘Mean anything to you?’ asked Hissop.

‘It may be nothing,’ Updark broke in, ‘but we can’t afford the risk of it being something.’

Dyer stared at the scribble. All at once, the sandpit reeled back. In among the small, imperfectly obliterated mounds, the same figure 7 and letter M.

They were watching him closely.

‘It looks like fried egg.’