'Ooh, Coupla ' she squealed, laughing at them. She often used words from her other language, and 'Coupla' sounded like cUpla, her word for twins. The twins were only four at the time. 'The two of you are rogues, so you are '

Kate and Dan are twins. When they were three, their mother Ana walked into the sea. Cormac, their dad, and Mrs Janey who looked after them while he was away, both tried their best to comfort them, but they still missed her.
The story begins when they meet the strange Ms Cliona, who lures them to the Underworld on dolphins, surrounded by a cocoon of white light.
Bewildered, they are forced to go through challenge after challenge on wondrous and enchanting islands, where they meet powerful ancient beings known as SIoga. Most of the SIoga are kind. Some are nasty. One, however, is like a mother to them, and they begin to wonder if Ana has really died, or if she is a powerful SIog who is 5,000 years old.