Prabir froze and watched her admiringly. There was no getting away from it: sometimes he felt the Madhusree-thing himself. The same sweet thrill, the same tenderness, the same unearned pride he saw on his father’s and mother’s faces.

He sighed heavily and swooned backwards into the water, touching bottom, opening his eyes to feel the sting of salt and watch the blurred sunlight for a moment before rising to his feet, satisfyingly wet all over. He shook his hair out of his eyes and then waded after Madhusree. The water reached his own ribs before he caught up with her; he eased himself down and started swimming beside her.

‘Are you all right?’

She didn’t deign to reply, merely frowning at the implied insult.

‘Don’t go too far.’ When they were alone, the rule was that Prabir had to be able to stand in the water. This was slightly galling, but the prospect of trying to tow a struggling, screaming Madhusree back to safety was something he could live without.

Prabir had left his face mask behind, but he could still see through the water quite clearly with his head above the surface. When he paused to let the froth and turbulence he was making subside, he could almost count grains of sand on the bottom. The reef was still a hundred metres ahead, but there were dark-purple starfish beneath him, sponges, lone anemones clinging to fragments of coral. He spotted a conical yellow-and-brown shell as big as his fist, and dived for a closer look. In the water everything blurred again, and he almost had to touch bottom with his face to see that the shell was inhabited. He blew bubbles at the pale mollusc inside; when it cowered away from him he retreated sheepishly, walking a few steps backwards on his hands before righting himself. His nostrils were full of sea water; he emptied them noisily, then pressed his tongue against his stinging palate. It felt as if he’d had a tube rammed down his nose.

Madhusree was twenty metres ahead of him. ‘Hey!’ He fought down his alarm; the last thing he wanted to do was panic her. He swam after her with long, slow strokes, reaching her quickly enough, and calming himself. ‘Want to turn back now, Maddy?’

She didn’t reply, but a grimace of uncertainty crossed her face, as if she’d lost confidence in her ability to do anything but keep swimming forward. Prabir measured the depth with one glance; there was no point even trying to stand. He couldn’t just snatch her and wade back to the shore, ignoring her screams, her pummelling and her hair-pulling.

He swam beside her, trying to shepherd her into an arc, but he was far more wary of colliding than she was. Maybe if he just grabbed her and spun her round, making a game of it, she wouldn’t be upset. He trod water and reached towards her, smiling. She made a whimpering noise, as if he’d threatened her.

‘Sssh. I’m sorry.’ Belatedly, Prabir understood; he felt exactly the same when he was walking on a log over a stream or a patch of swampy ground, and his father or mother grew impatient and reached back to grab him. Nothing could be more off-putting. But he only ever froze in the first place when someone was watching him, hurrying him along. Alone, he could do anything—casually, absent-mindedly—even reversing high above the ground. Madhusree knew she had to turn back, but the manoeuvre was too daunting to think about.

Prabir cried out excitedly, ‘Look! Out on the reef! It’s a water man!’

Madhusree followed his gaze uncertainly.

‘Straight ahead. Where the waves are breaking.’ Prabir pictured a figure rising from the surf, stealing water from each collapsing crest. ‘That’s just his head and shoulders, but the rest will come soon. Look, his arms are breaking free!’ Prabir imagined dripping, translucent limbs rising from the water, fists clenched tight. He whispered, ‘I’ve seen this one before, from the beach. I stole one of his shells. I thought I’d got away with it… but you know what they’re like. If you take something from them, they always find you.’

Madhusree looked puzzled. Prabir explained, ‘I can’t give it back. I don’t have it with me, it’s in my hut.’

For a moment Madhusree seemed about to protest that this was no real obstacle; Prabir could simply promise to return the shell later. But then it must have occurred to her that a creature like this wouldn’t be so patient and trusting.

Her face lit up. Prabir was in trouble. The water man lowered his arms and strained against the surface, forcing more of his body into existence. Bellowing from the pain of birth, baring glistening teeth.

Prabir turned a nervous circle. ‘I have to get away before his legs are free. Once you see a water man running, it’s too late. No one’s ever lived to describe it. Will you guide me back to shore? Show me how to get there? I can’t think. I can’t move. I’m too frightened.’

By now Prabir had psyched himself up so much that his teeth were chattering. He only hoped he hadn’t gone too far; Madhusree could gouge agonising furrows in his skin without the slightest qualm, ignoring his screams of protest, but she’d also been known to burst into inconsolable tears when anything else distressed him.

But she gazed at the water man calmly, assessing the danger. She’d been treading water since the creature appeared, and she’d already drifted around to face sideways. Now she simply leant towards the shore and started swimming, all difficulties forgotten.

It was hard work feigning panic without overtaking her, when her arms were about a quarter as long as his own. Prabir glanced over his shoulder and shouted, ‘Faster, Maddy! I can see his ribs now!’ The water man was leering angrily, already assuming a kind of eager parody of a sprinter’s crouch. Rocking back and forth on the tips of his splayed fingers, he dragged more of his torso out of the waves. Prabir watched as the creature inhaled deeply, driving water from his lungs through his glassy skin, preparing himself for the world of air.

Madhusree was beginning to slap the water open-handed, the way she did when she was tired. Prabir suspected that he’d be able to stand soon, but it didn’t seem right to intervene before he had to. ‘I’m going to make it, aren’t I? I just have to breathe slowly, and keep my fingers together.’ Madhusree shot him an irritable don’t-patronise-me look, and clawed the water in an exaggerated fashion before accepting his advice and powering ahead.

Prabir stopped dead and turned to examine their would-be pursuer. The last stage was always difficult; it was awkward trying to brace yourself as you dragged your legs up beneath you. Prabir closed his eyes and imagined that he was the water man. Crouching lower, forearms to the waves, he strained with his whole body until his muscles expelled a visible surge of brine. Finally, he was rewarded: he felt the warm air on the back of his knees, on his calves. His right foot broke free; the sole rested lightly on the surface, tickled by the choppy water as if each tiny crest was a blade of grass.

He opened his eyes. The water man was rising up, ready to spring forward, with just one foot trapped below the waves to hold him back.

Prabir cried out and started swimming after Madhusree. Within seconds, he knew the chase had begun. But he didn’t dare look back: once you saw a water man running, you were lost.

The violence of his strokes made Madhusree turn; she lost her rhythm and began to flounder. Prabir caught up with her as her head dropped beneath the surface; he scooped her into his arms and reached for the bottom with his feet. His toes hit the sand with Madhusree cradled safely against his chest.

Running through the water was nightmare-slow, but he pushed his leaden body forward. He tramped right over a bed of brown sea-grass, shuddering with each step; it wasn’t that the blades were sharp, or slimy, but it always felt as if something was hiding among them. Madhusree clung to him, uncomplaining, staring back, transfixed. Skin crawled on Prabir’s scalp. He could always declare that the game was over, there was nothing following them, it was all made up. In his arms Madhusree was a passenger, immune to the rules, but if he turned and looked for himself now, the simple fact of his survival would prove beyond doubt that the water man had never been real.