With each surge and each ebb, he had to wedge himself into new holes. Slowly, slowly. But not so slow that he would run out of air. His rapid breathing had been using it up at an alarming rate. The whirlpool’s roar was terrifying.

Finally he reached the end of the tunnel, where the current barely moved, a little place to rest before the cavern opened out into Waterfall Bay.

But his assailant was in here, too, curled up near the exit.

Breathe in, bubbles out.

The man saw Mackerel and fled, leaving behind a rush of bubbles from his flippers and regulator.

Mackerel struggled to swim with his single fin. He didn’t have a chance of catching him.

Once he was out of the tunnel and into Waterfall Bay he was swept up again. He breached the surface. He had to orient himself, but the crosshatch waves were like a washing machine. He struggled to hold his mask and regulator in place.

This wasn’t the way out he had planned – the water was much wilder here – but he had to track down Davey’s killer. He knew a place where he could leave the water and he did his best to head that way, ducking under the waves rather than over the top of them. He was heading to the near side of the bay, where there was a shelf of rock which sloped up out of the sea.

But when one wave caught him by surprise and lifted him high in its crest, he saw his destination – his assailant was there, clambering out of the water onto it.

This man had to be a local, to know about that shelf.

Mackerel dived back under and swam, lopsided and kicking against the current, mostly in vain. A heavy wave crashed right above him – it must have been a white wall – and his regulator was wrenched from his face.

Another giant wave forced him down, pulling at his remaining fin. He unbuckled it, the calm he’d felt earlier quickly evaporating, as he was smashed down again and held under.

His mask gone now too, he grabbed his spare regulator – calm breaths – and put air into his BC, shooting himself to the surface.

He kicked and kicked, straining already depleted muscles, trying to get closer to the shelf, where the cross-waves couldn’t push him back out to sea. He had the regulator, he had his BC, he was fine . . .

He searched for the shelf, panic threatening to swamp him. And then, finally, he caught the right wave, as the cold water pulled back and then threw him forward.

He slammed against the shelf. Scrabbling, he clung his fingers to the rocky clefts as the water tried to suck him back. Exhausted, he slipped out of his BC vest, relieved to feel the weight fall off him, letting all of his gear go.

Where was his assailant? He was nowhere to be seen.