Aquina is a mermaid. She has always felt different from the other merpeople on Rulantica. Shortly after her twelfth birthday, she finds out something incredible: she has a twin brother. Mats, a human boy! And he is in great danger. For Aquina, there's no holding back: she has to find her real family before it's too late. Ever since he was found on the beach as a baby, Mats has been brought up in a children's home. He has always been afraid of the sea and of water. What he doesn't know is that he'll soon be diving into the biggest adventure of his life! By coming together, Aquina and Mats are fulfilling a centuries-old prophecy by the Nordic gods – this could be a chance to save the island world of Rulantica, but it could also mean its total destruction ...