Once upon a time, there was a boy who wanted to be a merman. He knew it was supposed to be the other way around — a mermaid wanting to become human — but the heart what it wants, and he wanted to own a tail, breathe underwater, and talk to fish.

After years of longing and days walking along the windswept grey harbour and unforgiving grey beach of his miniature coastal town, a chance encounter brought him a step closer to achieving his only desire. As he trudged home from school along the shore, boots hitting pebbles a hair's breadth away from the water, a voice interrupted his thoughts.

'Child, will you do me a favour?'

He whirled around and found himself nose to nose with a… a crone?

'I'm a sea hag, dear,' said the creature soothingly as if she (it?) could read his thoughts. 'You know, a standard mythological being of the ocean with various magical powers and a penchant for deals with humans.' Her chalky skin was speckled and cracked like a rough seashell, and she wore a dress that looked like it had been woven from trawler nets. Her eyes were like a shark's: completely black and unblinking.

The boy noticed he was cold; all the hairs on his arms stood up and when he inhaled he could taste rotting fish.

'Hello,' he tried. 'It's… Nice to meet you.'

'You too, dearie,' chuckled the hag. 'Now, about that favour. I need you to do something for me. I'll give you a handsome reward,' she added after a beat. 'Anything your heart desires.'

'If you're offering a reward, it's not a favour, is it?' the boy pointed out. 'It's a deal.'

'I suppose you're right,' the hag sniffed. 'Well, do you want to hear it or not?' The boy considered. Deals were hard to come by in this part of the world. Optimism was harder.

'Go on then.'

'I can't leave the sea, dearie. It's an occupational hazard. And I need something returned to me that was stolen by upstart sailor many years ago.'

'What is it?' the boy asked.

'A piece of coral,' the hag replied. 'It was a part of my collection of tropical plants, and he stole it on a dare one day.'

'Rude,' agreed the boy. 'Do you know where it is?'

'Oh yes,' breathed the hag. 'It's in a museum. Your town's museum. He donated it on return from his voyage. Didn't even want to keep the stolen property, no, just wanted something shiny to bring home.'

'What will you do for me if I steal it back?'

The hag grinned, revealing approximately three and a half teeth. 'I can brew you a potion that will free you from your earthly bounds, human, and give you the life of a sea dweller you've wasted nights dreaming about.'

That was a melodramatic way of putting it, he thought, but why not.

'All right,' he said after a moment moment's consideration, just to be sure. 'I'll meet you back here this time tomorrow.'

The town's four-room museum (dedicated to fishing, naval exploits, marine life and the region's history respectively) was conveniently situated not five minutes' walk from the beach – although around here, most things were not five minutes' walk from the beach.

The only people who visited the museum – and the rest of the town, come to that – were tourists who had been on their way to a sunny corner of the coast and missed the turn off. Even seagulls decided not to take up permanent residence in the area (the young people who lived there were of a similar mind). Entering the museum was easy — it was open to the public six days a week but unstaffed for five of them, and entrance was fifty pence. Not bad for the fulfilment of one's lifelong ambition. Finding the coral was easy, too — it was one of five exhibits in the marine life room. None of the exhibits were actually alive. Perhaps fifty pence was a rip-off after all.

The boy approached the coral, now bleached and perched on a pedestal, and allowed himself a moment of trepidation. This is it, he thought, from this moment my life is going to start —

'Don't touch that,' said a voice. The boy nearly had a heart attack as for the second time in an hour as he turned to see the last person he would have expected in a museum on a weekday afternoon: a boy from his science class. The only notable boy from his science class, in fact, a star pupil destined to move to a faraway city and visit on seasonal holidays.

'Jimmy. James. Hi,' he remembered too late that James was only called Jimmy in his head. He looked like a Jimmy. The boy had only ever seen Jimmy across a classroom; in person his eyebrows were slightly asymmetrical and his nose slightly crooked. He wore what looked suspiciously like a handknitted pullover over his reasonably fashionable jeans.

'Hello,' said Jimmy amicably. 'It's Roger, isn't it?'

'Yes,' the boy said uncomfortably. Perhaps now you can see why we've called him 'the boy' until this point. When you think of mer-people, you just don't think of Rogers.

'Are you here to steal the coral?'

'Maybe,' Roger replied. Jimmy stuffed his hands in his pockets.

'That coral carries a curse. If you touch it, you'll never be able to go near the sea again.'