Cassie Morgan needs to earn her Sapling pin to get one step closer to passing her Witch's Licence. To do this, she's set a challenge of training a beginner witch by the Hedgewitch - but the newest member of her coven is not what she expected. Meanwhile, the wicked Erl King is luring witches up and down the country to his side, with promises of unimaginable power. Those who stand against him are running a deadly risk. When a letter arrives from Cassie's cousin Sebastian, desperate for help to solve an intriguing mystery, she travels to his home of Porthmorven with her friends, both new and old, to help work out what's going on. But Porthmorven is no ordinary place, it's a Faerie border town, like Hedgely, and it has its own protector: the elusive Seawitch. But she is nowhere to be found and Cassie is determined to help the people of Porthmorven however she can. Can Cassie use all of her training - along with help from her friends - to solve the secrets of the sea and foil the Erl King's plans once and for all?