Halfway through his fifth lap, he spotted Grant approaching. She called out to him jokingly, ‘I thought you’d been kidnapped.’

‘Sorry. I got carried away.’

‘I don’t blame you. It’s unbelievable.’ They trod water over an outcrop of branched red coral, festooned with anemones and swarming with tiny bright fish—all at least six metres below them, but the details were so sharp that they might have been looking down through air.

Prabir felt a sudden urge to come clean with her; whatever the significance of the butterflies turned out to be, he was tired of having the deception between them. He’d proved himself useful to have around, even if it was more as ad hoctechnical assistant and general dogsbody than cultural liaison. And surely she’d understand his reluctance to reveal the whole family history to a stranger.

He struggled to find a place to start. ‘Were your family excited by the news last night?’ He hadn’t eavesdropped; she’d been talking to her son right in front of him as he’d gone out on to the deck to sleep.

Grant frowned. ‘News? You mean the pigeon sequences? I couldn’t tell them about that; there’s a confidentiality clause in my contract.’

Prabir was shocked. ‘But you—’

‘And you mustn’t mention it to anyone, either. Especially not your sister.’

Prabir was about to retort that he wasn’t bound by any contract, but it didn’t seem like a good idea to drive home the point that she’d been unwise to confide in him.

He said, ‘Whatever happened to scientists sharing data?’

‘Welcome to the real world.’

‘And you’re happy with this?’

‘Delirious. I love being gagged.’ Grant plucked irritably at something crawling up the arm of her T-shirt.

‘Then why did you do it? Why did you sign the contract? Couldn’t you have joined the university expedition instead?’

‘I’m not an academic. Everyone on that boat is being paid a salary from somewhere—student slave labour like your sister excepted. In the unlikely event that they’d let me on at all, I would have had to pay them for the privilege. I enjoy what I do, but I’m not in it for charity. I have a family to support.’

Prabir wasn’t about to do a post-mortem on anyone’s career choices. ‘How long does it apply? The gag?’

‘That depends. Some things might be cleared for publication by the lawyers in a couple of months. Others might take years.’

It came to him suddenly that his parents had published nothing in all their years on the island. They’d taken money from Silk Rainbow. They must have made the same kind of deal.

Grant frowned. ‘Are you OK?’

‘Just a stitch.’

‘You’re not planning to quit on me in disgust?’

‘Hardly.’ It shouldn’t have stung so much. They’d made one small compromise in order to do something that otherwise would not have been done at all. When had he started thinking of them as flawless, immaculate?

Grant started back towards the boat. Prabir called after her, ‘New rules, though. First one out of the water cooks breakfast.’

Grant had chosen six small islands from which to gather samples, lying in an arc that ran south-east from the Bandas to the Kai Islands. All were uninhabited, unless they had settlements so small that they’d escaped the notice of the official cartographers. The third was just seventy kilometres north-east of Teranesia, slightly closer than the Tanimbar Islands to the south; if it had been on the maps when Prabir was a child, he and Madhusree might have ended up stranded there.

When he’d joined Grant in Ambon, he’d imagined himself somehow ‘steering’ her towards the source of the mutations; fat chance of that, but the route she’d picked would already take them about as close as he wanted to get. He could only hope that whatever the biologists’ expedition had discovered was drawing them in the same direction; it seemed naive now to think that Madhusree—lowest of the low in the academic pecking order—could have swayed a boatload full of experts with their own theories and agendas.

They sailed out of Banda Harbour early in the afternoon, and it was close to sunset when they arrived at the first of the islands. They dropped anchor a hundred metres from shore and spent the evening recuperating, drawing entertainment off the net. To Prabir’s amazement, Grant turned out to like Madagascan music as much as he did, and she knew all the esoterica better. After a while he stopped trying to compete with her at naming performers and recordings, and just let her dazzle him with her erudition.

Grant winced suddenly. ‘Quarter to ten! I promised Michael I’d call him in his lunch hour.’

Prabir went out on deck to give her some privacy. He sat perched on the guard rail at the stern of the boat, swaying slightly to keep himself upright, the sound of the valiha still playing in his head.

If he agonised over it, he knew he’d never do it. He pulled out his notepad and hit three buttons in rapid succession.

Felix grinned up at him from the screen. ‘How’s it going?’

Prabir shrugged. ‘It was strange being back at first, but I’m getting used to it. How’s work?’

‘Dull beyond words. I’m disgusted that you’d even ask. Any sign of Madhusree?’

‘Not yet. I think we’re both heading in the same direction, but it’s going to be a matter of luck whether I catch up with her or not.’

Felix said tentatively, ‘I could always call her and tell her you’re on your way. It’s not as if she could really pressure you into turning back now, even if she wanted to. And she might take the whole thing better if she was forewarned.’

‘I don’t think that would be a good idea.’

‘Compared to what? Arriving unannounced?’

Prabir thought seriously about the suggestion. But why risk alienating her, when there was still no guarantee that their paths would actually cross? He said, ‘Don’t worry about it. If we meet, we’ll sort it out. If we don’t, I’ll confess everything once we’re back in Toronto, and she’ll just laugh and forgive me on the spot.’

He recounted as much as he could about the Bandanese pigeons; Felix seemed neither surprised nor offended that he couldn’t be let in on the sequencing results. They talked for almost half an hour, until Felix had to go and refill his pipetting robot’s reagent tanks.

When the window closed and Prabir looked up, his eyes still adapted to the brightness of the screen, he felt unspeakably strange. It wasn’t just a pang of loneliness; he wasn’t sure that it had much to do with Felix at all. It was the connection breaking, the image fading, the whole illusion collapsing in front of him, leaving him with nothing but darkness and the mechanical rocking of the sea.

He sat on the railing, watching Grant smiling and laughing in the cabin, and waited for the feeling to pass.

They circumnavigated the island, probing its fringing reef with sonar until they found a safe approach to a small sandy beach. Grant anchored the boat in a metre of water, and they waded ashore. Prabir looked down at the fine, bone-white sand with a jolt of recognition, but he let the feeling wash over him, neither fighting it nor pursuing it to its source.

He found some shade and sat to pull his boots on, squinting back at the sunlit water. Silver on turquoise, the view was indistinguishable from one he’d seen a thousand times before. The memory went deeper than vision: as he tightened his laces he grew aware of a disconcerting ease in his limbs, an assured and unselfconscious physicality beneath the fading ache from the plantation. A few laps in Banda Harbour could hardly have restored him to childlike resilience, but on some level his body still carried a trace of what it had once felt like to swim in this sea every day.

Grant said, ‘Are you ready?’ She gestured at the mine detector clipped to her belt. Prabir hit the self-test button on his own device; it chimed reassuringly and flashed a green light, whatever that was worth.