Sereia, a magical mermaid, saves her younger sister and inadvertently threatens her brother Triton's claim to the throne. In retaliation, Triton curses Sereia, transforming her into a human and banishing her from their underwater kingdom with a dire warning: if she ever returns, she 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.
As Sereia adjusts to life on land, she learns that much of what she was taught about humans may be wrong. Despite needing to keep her true identity a secret, Sereia finds herself drawn to her savior, and her feelings for him deepen. Determined to make a life for herself, Sereia channels her skills into joining the Atlantean army, aspiring to become the respected warrior she always desired to be. During her training, she discovers the existence of a powerful trident, rivaling her father's, hidden within Atlantis. This revelation presents Sereia with a potential path back to the sea and her magic.
However, Sereia is torn between the pull of revenge and the possibility of a new life on land, especially under the watchful eye of a vengeful enemy within her ranks. Ultimately, when a friend’s fate hangs in the balance, she faces an agonizing decision: risk being burned at the stake as a witch or transform into a monster by returning to the sea.