Thomasina And Anne Are Looking For Adventure When Their Friend, Henry, Disappears – Snatched By A Mysterious Creature, And Dragged Into The Depths Of The River Thames. Determined To Save Him, The Girls Venture Into The Deep, And Discover The Enchanting Free Waters: A Vast, Glittering Underwater City, Home To Mermaids And Sirens. But The Free Waters Are Threatened By An Evil Sea Queen. It Is She Who Has Henry– And The Girls Must Complete Three Treacherous And Impossible Magical Tasks If They Are To Save His Life … An Unputdownable And Exciting New Tale From The Bestselling Author Of The Miraculous Sweetmakers: The Frost Fair.