Siren Phillips has lived her life thinking she's an ordinary girl, in an ordinary town. On the eve of her twelfth birthday, however, she learns that she is far from ordinary. Destined to turn into a Mermaid at the age of twelve, Siren must struggle with her new reality, saying good-bye to her mother and friends, while she transitions into the water. To make matters worse, a group of hunters are trying to catch her. She soon becomes friends with Adam Wilts, a new boy at school, who she turns to for help. He agrees to help her, but finds himself caught in the middle, when it's his own father who is after Siren. When Siren's mother is taken, the town must rally behind her, and help her make a peaceful transition into the water, before the hunters can find out where she is.

　　剧情延续了上一部，玛尔（德芙·卡梅隆 Dove Cameron 饰）和本（米切尔·霍普 Mitchell Hope 饰）排除了千难万险，最终走到了一起。自此之后，玛尔开始学习如何成为一名合格的公主。但随着时间的推移，玛尔渐渐意识到，现在的生活虽然富足，可是却并不能令她感到快乐，她必须在过去和未来之间做出选择。
　　一次偶然中，玛尔和本之间爆发了一场激烈的争吵，一怒之下，玛尔回到了失落之岛，再度见到了自己过去的好友乌玛（琪娜·安妮·麦克兰 China Anne McClain 饰）、哈里（汤马斯·多赫提 Thomas Doherty 饰）和吉尔（迪伦·普莱费尔 Dylan Playfair 饰）。回到了失落之岛的玛尔换回了自己曾经的服装和发色，直到此时她才总算感觉到做回了真正的自己。