A century ago, the human-mer treaty was established to protect the mer from the humans while they guided ships through storm-plagued waters. Now, Aila and her cousin, Persephone, act as representatives for their grandfather, King Gaspar, to the human world. The mermaids share a close bond with the Prince Jarek, but when Alia catches Persephone trying to siren him into the waters, she must work to protect her friend from her cousin without hurting the human-mer relationship—something Persephone’s mother won’t tolerate. War is brewing and Aila and Persephone are caught in the middle of a battle they never saw coming—one that will last for another century. Every war has an origin...she just never thought it would be her.

War has hovered around the kingdom of Scylla for generations ever since the original sirens left the mer collection generations ago after nearly drowning the human prince. Over the years, select mermaids from the royal bloodline have been trained as spies to work for the reigning kings and queens, keeping the collection safe from sirens and humans. Celena and her partner, Merrick, work covertly for the royals—not even her twin brother knows. When they discover the sirens have broken through the barriers the mer set up to keep the sirens out, Celena and her friends must race to the old kingdom of Metten to stop them from starting a war within their borders. When she’s dragged to the surface, Celena realizes that the war above the waters is as deadly as the one below the waves—and sacrificing herself may be the only way to protect her family. The Siren Wars have only just begun.

The sirens have ripped Celena away from her collection of mer, leaving her exposed to the dangers of the open seas. As she fights to save her sister, the royal mermaid is forced into deadly encounters with anything that gets in the sirens’ way, taking the brunt of the trauma as she tries to protect her little sister and escape the sirens’ revenge. She quickly learns that the attack on the palace in Scylla wasn’t the only thing Tarni and Nir had planned—they intend to use Celena and her sister for an even darker purpose—starting a battle with the humans. When Merrick, Caspian, and Llyr try to free Celena and Coralie, they disrupt the sirens’ plans and find themselves in the middle of a war with both their enemies under the sea and the humans above the waves. As the collection closes in to aid in the rescue mission, they find themselves in a deadly siege above the surface that can only end in destruction. The Siren Wars are nowhere near over.

Inside the human palace, Celena is faced with a choice—cooperate or die. Separated from Merrick and the other mer, her only choice is to sway the human prince into befriending her despite his father’s resistance to create a new treaty with the mer. When her twin and the collection find Celena and Merrick, they create a plan to escape, but dangerous advances from above threaten to destroy their chance at survival. Celena and her friends find themselves thrust into a new side of the war—one where they have to side with the humans and work to help save them before it’s too late for anyone to survive this new battle. She may be fighting for her enemy’s safety, but the royal humans might kill her anyway. One way or another, the Siren Wars will finally end.