When magical mermaid Sereia saves her little sister and overshadows brother and the sea's rightful heir, Triton, the position of next ruler of the sea is in question. Determined to keep his throne, Triton curses Sereia, transforming her into a human and stripping her of magic. Banishing her from their underwater kingdom, he gives her a final warning: if you should ever return, you will become a monster.

Left for dead, Sereia washes up on the shores of Atlantis, where she is discovered by a kind merchant with a tragic past. Patient and charming, he helps her build a life on land; she realizes everything she was taught about humans may have been wrong. But legends are powerful forces, and mermaids are burned alive for their magic by humans who fear their power. Sereia is forced to keep her true identity a secret, even as her feelings for her savior deepen.

Channeling her skill with a blade, she finds a place within the ranks of the Atlantean army, finally giving her the chance to become the respected warrior she always desired. During her training, however, she discovers the legend that a trident of equal power to her father's exists and is within her grasp in Atlantis. With a way back to the sea in her grasp, she wavers between the pull of revenge and the possibility of love on land, all under the hateful eye of a vengeful enemy within the army's ranks. But when the fate of a friend is in the balance, she must make the hardest decision of all: be burned at the stake as a witch, or turn into a monster should she return to the sea.