A devastating flood has torn through the worlds of Air and Ingo, and now, deep in the ocean, a monster is stirring. Mer legend says that only those with dual blood 'half Mer, half human' can overcome the Kraken.

Sapphy must return to the Deep, with the help of her friend the whale, and face this terrifying creature, and her brother Conor and Mer friend Faro will not let her go alone.