Out in the open sea, it's kill or be killed. No one knows that better than Kaelyn. For the past year, Princess Kaelyn of Avalon has disguised herself as a man, Captain Kae, and led her crew into tumultuous waters to eradicate sirens on a journey fueled by revenge for the death of her brother. When they return home and experience a fatal siren attack on Avalon's harbor, Kae sets sail again to destroy the sea demons once and for all.

Aqeara is a siren warrior of Meyrial, an underwater kingdom hidden from humans. When her negligence during the Avalon harbor attack causes the death of Meyrial's princess, she accepts the help of a sea witch to overturn her banishment. Aqeara is given a human body and has until the next full moon to carve out Captain Kae's heart in exchange for a spell to bring the dead princess back to life.

When Kae's and Aqeara's paths cross, they fall into a whirlwind romance, complicating their respective plans. Kae must decide whether her desire for revenge against sirens overrides her feelings for the woman she's falling for, and Aqeara must choose between resurrecting the dead princess or sparing Kae's life.