A jewel thief with secrets. A warrior with everything to lose. A heist that could change the fate of Atlantis.

Fiona Campbell lives a double life—fairytale illustrator by day, legendary thief known as the Scarlet Ninja by night. Her next target: the Siren, a priceless gem housed in the Tower of London. But she's not the only one after it.

Christophe, Atlantean warrior and protector of the lost treasures of Poseidon, needs the Siren to restore his god's shattered trident. To retrieve it, he must join forces with the one woman who tempts him to break every rule.

But they aren't the only ones in the game. A deadly evil stalks their every move—one that could destroy their fragile trust... and ignite a war between worlds.