The tale starts in a flashback, the heroine Sapphire is remembering her lost father. Her father, Mathew Trewhella, disappeared one day, and never came back. Sapphire is haunted by the fact that he didn't come back. Many people said that he had run away with another girl after falling for her but Sapphire and her older brother Conor do not believe that.
Then one day, Conor disappears. Sapphire fears that the same thing that happened to her father is now repeating itself. Usually Conor tells her where he was going; he doesn't just disappear. Eventually becoming too worried to continue hanging around the house, Sapphire sets out in search of her brother. She finds him on the cove rocks; he's been swimming and is now talking with a mermaid named Elvira. After saying goodbye, Conor goes to greet his sister and is surprised that the time passed much more quickly than he had expected. At first Connor does not believe that he was swimming for seven hours, but is forced to believe by his sister, who is frightened that her brother found out something he did not tell her about.
A few days later, Sapphire gets a longing to go out into the sea with her brother. She goes, unheedful of past warnings from her brother ringing in her ears. She suspects that Conor is there, so she calls out to him. He doesn't come. Then she hears a song...a hauntingly familiar song...the song her father used to sing to her. Sapphire goes underwater, and meets her own mer friend who takes her on many adventures. After staying in Ingo for a whole night, Conor explains to Sapphire that every time you go to Ingo, every time you get deeper there, it takes more Earth time than you would have thought. Sapphire waves off his warnings ("Don't go there without me, you'll lose track of time!") and plans to meet Faro.
Sapphire starts acting oddly. She puts salt in her water, saying that "it tastes better that way." She also asks for anchovies, and doesn't eat her Mum's homemade sausages. Conor gets worried and the novel proceeds as Sapphire and Connor encounter and complete extraordinary adventures that no one can dream about.