For the longest time the Merrow didn’t move. Cat could see a whole host of emotions play out on the creature’s face, including one she recognised all too well – longing for something she missed badly. Cat almost had her. She had to make one last push.

‘Merrow of the Sea. I promise, on the blood of my ancestors, I will find a way to return you to the sea or die trying.’

The Merrow hissed, ‘You would make a blood oath?’

‘Um, yes?’ Cat said, unsure of what she had just done. She had been channelling her inner space captain and had said something that sounded suitably heroic. Now she was beginning to think she had made a mistake.

Never make a promise to the Sídhe, said the voice in her head, which sounded suspiciously like Granny. They’ll pursue you to the ends of the earth until you fulfil it.

‘Well,’ Cat whispered to herself. ‘I’ll just have to keep my word then.’

‘Two hundred years,’ said the Merrow with such sadness in her voice that Cat almost wanted to reach out and give her a hug. But then she spied those sharp, dirty claws and thought twice about it.

Don’t hug the creepy mermaid, she thought in a voice which was entirely her own.

‘Two hundred years I’ve been trapped in this puddle. Do you know what torture it is when it dries up every summer and traps me below the earth, wishing to feel the moonlight on my face? Do you know how it feels to be separated from your sisters? To be too far away to hear the song of the waves? I will take your bargain, child. Get me home and you can have your friend back.’

‘Friend first,’ Cat said with a glance to the ghost-girl and the dozens of other souls now crowded around her. ‘Along with all of the souls of the children you’ve taken over the years.’

A ripple went through the ghost-children, and she felt them move closer.

‘Impossible!’ said the Merrow and clutched Shane so tightly to her he began to turn purple. Cat had to act quickly.

‘You don’t need them any more,’ she said, ‘not if you’re going home. And I won’t leave without them. Now what’s it to be?’

The silvery forms of the ghost-children pressed in closer now, their chance at freedom so close they could almost taste it.

The Merrow was like a cornered animal. She looked at Cat with venom in her eyes.

‘It’s a deal,’ she said, and Cat felt her whole body exhale. ‘But bear this in mind – you have made a blood oath to me and if you fail to keep your end of the bargain it will be you I take as my prize and rest assured, I will not part with you so easily.’

Cat nodded, her mouth too dry to say anything else.

With slightly shaky hands, she screwed open the top of the empty cola bottle and held it out in front of her. Then, a lot of things happened at once. The Merrow raised her arms and began to swirl like a maelstrom, dropping Shane. As soon as he hit the surface, the souls of the ghost-children erupted out of the lake like a tidal wave.