After facing down Father Winter and escaping the Frost Fair, Thomasina and Anne have achieved their dream. Without needing any wishes at all, they're running their own sweetshop.

But magical creatures and otherworldly beings aren't quite done with them yet, and they soon find their friend Henry kidnapped by an evil Sea Queen and dragged down into the depths of the River Thames.

Determined to save him, the girls venture into the deep and discover the enchanting world of the Free Waters: a vast, glittering underwater city, home to mermaids, sirens, and selkies. But the journey to rescuing Henry is more dangerous than they could have imagined, and the Sea Queen a powerful enemy--one who might be even stronger than Father Winter himself.