Munroe shook both hands, putting no power into it: what had he to prove?

‘Well, the wind’s dropped a bit,’ Munroe said, rubbing the grey stubble on his chin. ‘So I suppose that’s all right.’ He turned to his wife. ‘Meg, what about some bread and ham for the lad?’

Mrs Munroe nodded and silently set about her work, while her husband readied himself. He found oilskins for all of them and waterproof boots for himself, by which time a parcel of sandwiches and flask of tea sat waiting. Rebus stared at the flask, knowing Jack was doing the same, both gasping for a drink.

But there was no time for that. They were off.

It was a small boat, freshly painted and with an outboard motor. Rebus had had visions of them rowing across.

‘There’s a jetty,’ Briony said when they were underway, rising and falling against the choppy water. ‘A ferry usually takes visitors across. We’ll have a bit of a hike, not much.’

‘It’s a bleak spot to choose,’ Rebus yelled above the wind.

‘Not that bleak,’ she said with the ghost of a smile.

‘What’s that?’ Jack said, pointing.

It stood on the edge of the island, next to where sloping strata of rock eased down into the dark water. Sheep grazed on the grass around the structure. To Rebus, it looked like some gigantic sandcastle or upturned flower-pot. As they got closer he saw it had to be over forty feet high, maybe fifty feet in diameter at its base, and was constructed from large flat stones, thousands of them.

‘Mousa Broch,’ Briony said.

‘What is it?’

‘Like a fort. They lived there, it was easy to defend.’

‘Who lived there?’

She shrugged. ‘Settlers. Maybe a hundred years BC.’ There was a low-walled area behind the broch. ‘That was the Haa; it’s just a shell now.’

‘And where’s Jake?’

She turned to him. ‘Inside the broch, of course.’

They landed, Munroe saying he’d circle the island and be back for them in an hour. Briony carried the bag of provisions, and struck out towards the broch, watched by the slow-chewing sheep and a few strutting birds.

‘You live in a country all your life,’ Jack was saying, the hood of his oilskin up to protect him from the wind, ‘and you never even know stuff like this is out there.’

Rebus nodded. It was an extraordinary place. The feel of his feet on the grass wasn’t like walking across lawn or field; it was like he was the first person ever to walk there. They followed Briony through a passageway and into the heart of the broch itself, sheltered from the wind but with no roof to protect them from threatening rain. Munroe’s ‘one hour’ was a warning: any later and they’d be in for a rough if not dangerous crossing.

The blue nylon one-man tent looked incongruous pitched in the broch’s central court. A man had risen out of it to hug Briony. Rebus bided his time. Briony handed over the bag of tea and sandwiches.