Serafina, Neela, Ling, Ava, Becca, and Astrid, six mermaids from realms scattered throughout the seas and freshwaters, were summoned by the leader of the river witches to learn an incredible truth: the mermaids are direct descendants of the Six Who Ruled—powerful mages who once governed the lost empire of Atlantis.

The ancient evil that destroyed Atlantis is stirring again, and only the mermaids can defeat it. To do so, they need to find magical talismans that belonged to the Six. Serafina believes her talisman was buried with an old shipwreck. While researching its location, she is almost discovered by a death rider patrol led by someone familiar. The pain of seeing him turned traitor is devastating.

Neela travels to Matali to warn her parents of the grave threat facing their world. But they find her story outlandish; a sign that she needs to be confined to her chamber for rest and recovery. She escapes and travels to Kandina, where her talisman is in the possession of fearsome razormouth dragons.

As they hunt for their talismans, both Serafina and Neela find reserves of courage and cunning they didn't know they possessed. They face down danger and death, only to endure a game-changing betrayal, as shocking as a rogue wave.

Serafina, daughter of Isabella, Queen of Miromara, has been raised with the expectation—and burden—that she will someday become ruler of the oldest civilization of the merfolk.

On the eve of the Dokimí ceremony, which will determine if she is worthy of the crown, Sera is haunted by a strange dream that foretells the return of an ancient evil. But her nightmare is forgotten the next day as she diligently practices her songspell, eagerly anticipates a reunion with her best friend, Neela, and anxiously worries about Mahdi, the crown prince of Matali, and whether his feelings toward her and their future betrothal have changed. Most of all, she worries about not living up to her mother's hopes.

The Dokimí proceeds, a dazzling display of majesty and might, until a shocking turn of events interrupts it: an assassin's arrow wounds Isabella. The realm falls into chaos, and Serafina's darkest premonitions are confirmed. Now she and Neela must embark on a quest to find the assassin's master and prevent a war between the mer nations. Their search will lead them to other mermaid heroines scattered across the six seas. Together they will form an unbreakable bond of sisterhood as they uncover a conspiracy that threatens their world's very existence.