‘Friends?’ said the Merrow with a scoff. ‘Why should I need friends? I have my children.’ She gestured out across the lake and once again the water lit up with dozens of glowing figures suspended just above the surface. ‘And now I have one more to add to my collection.’

‘But it’s not the same, is it?’ said Cat desperately. ‘You can’t force people to be your friend, they’ll only end up hating you. Trust me, I know.’

The Merrow sneered and Cat could tell she was losing interest in their discussion. If she went back under the water, it was all over and Shane would be lost forever. Cat had to make her stay.

‘What if I told you I could help you get back home, back to your real friends?’ she blurted. ‘Back to your family?’

The Merrow regarded Cat with intense interest.

‘How?’

Cat felt her mouth go dry. She hadn’t quite thought this far ahead.

‘Well, through my powers of course. I … I have powers you can’t even imagine.’

The Merrow threw back her head and began to laugh. It was not a kindly sound. ‘Child, I swam the seas back when the Tuatha Dé Danann themselves roamed this land. I have witnessed countless kingdoms rise and fall. What powers could one such as you possibly have that I have not seen thousands of times over?’

‘Wait just a minute,’ she said as an idea came to her, ‘and I’ll show you.’

She waded back to shore, cringing as she turned her back on the Merrow. She was sure that any second she would feel those cold, clammy fingers closing around her neck.

‘What are you doing?’ hissed the ghost-girl who had floated up beside Cat. ‘You don’t have any powers!’

‘Shh,’ Cat said, ‘keep it down, will you? Just keep an eye on the Merrow for me. I need to get something.’

The ghost-girl looked doubtful, but she nodded.

Cat found her schoolbag where she had discarded it a few feet from the water’s edge and rummaged through it until she found what she was looking for – the half-empty bottle of cola. She hunched over it so the Merrow wouldn’t see what she was doing, unscrewed the cap, and poured out the remainder onto the grass.

Then, with as much reverence as she could muster, she held the bottle aloft and walked back to the water’s edge.

‘Behold!’ Cat said, ‘the mystical bottle of … Pep-Sea.’

The ghost-girl gaped in surprise. Cat would have kicked her if she thought her foot wouldn’t just go straight through.

‘I have not heard of this bottle,’ said the Merrow with narrowed eyes. ‘And I know all the treasures of the ancient world – from the Stone of Fál to the Cauldron of the Dagda. Don’t you dare try to trick a Merrow, girl. Let me assure you, it will end badly for you.’

‘Oh, but you wouldn’t have heard of this one,’ Cat said, thinking on her feet. ‘Because … because it was forged far away, in a land across the sea. In fact, that’s why I know it can help you return home. It has the power to … to transport whatever is inside of it across great distances. If you agree to get in the bottle, I can use it to take you to the sea.’