While Ladybug, Cat Noir, and Rena Rouge are handling overrun domestic trouble, Kim and a talented swimmer, Ondine, are at the swimming pool when Ondine tries to send a love message to Kim, but he is so dense he leaves without reading it. Sensing Ondine's broken heart, Hawk Moth finds her a perfect prey to akumatize and turns her into Syren, a mermaid supervillain flooding the city. Water is surging all over Paris and Syren captures Kim to rule the underwater city with her prince. Meanwhile, Adrien is expressing doubts being a superhero since Ladybug is keeping too many secrets from him lately, then he and Master Fu officially meet. Fu has applied the both heroes a new aquatic power that modifies the kwami to breath underwater. Gabriel also learns about the hidden power of the kwamis and he wants these for his own kwami, but since only a Guardian of the Miraculouses knows the secret formula, he is determined to find the Guardian, wherever he is.