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.