Emily Windsnap, an ordinary girl on land and a magical mermaid in water, is overdue a break from excitement - as her exhausted best friend Shona keeps reminding her! So, promising Shona she'll stay free of trouble and just 'swill out' ('chill out' for mermaids), Emily invites her best friend and her boyfriend Aaron along for a relaxing family holiday in the sun . . . But it seems adventure is hot on her heels - and her tail.

Soon, Emily discovers a hidden world beyond the falls of mysterious Forgotten Island - but it's not quite the paradise it seems. With a curse hanging over the island and a catastrophic disaster looming, Emily finds herself suddenly entangled in an ancient Prophecy. She must embark on a dangerous mission to seek the legendary giant the islanders believe will save them all, before it's too late.But how can Emily save the day when rifts are forming between her and the very people she needs by her side? Must promises always be kept? And what happens when they are broken?