In a coastal town where fish-people known as mermaids live alongside humans, a special medicine allows them to temporarily transform their tails into human legs to attend regular schools and integrate into society. Wakasa Otomi, a human high school girl with an intense enthusiasm for all things related to these fish-people, is thrilled by this new reality. Her passion leads her to befriend a new transfer student, Tsutsumi Anko, a lantern fish mermaid who is deeply self-conscious about being a deep-sea creature. Unlike the colorful and sleek tropical fish, Anko believes her tail is ugly and her unique traits, like the bioluminescent lure on her forehead, are more monstrous than charming.

The story unfolds as a light-hearted slice-of-life comedy centered on Anko and Otomi navigating school life, where the biological quirks of various sea creatures create unusual social situations. Each mermaid character exhibits traits of their real-world counterpart. The supporting cast includes Touna Suzuki, a showy and competitive betta fish mermaid who initially bullies Anko but reveals a tsundere loyalty; Akame Fukuda, a puffer fish mermaid whose poisonous skin has isolated her, leading her to develop a chunibyo (delusional) persona as a dark magician; and Una Kabayagi, an eel mermaid who struggles with her tendency to secrete slime when nervous. A notable recurring situation involves Anko’s glowing lure, which can attract other fish-people against their will or be used as a flash of light, causing chaos in the classroom.

Throughout its narrative arcs, the manga explores Anko’s journey to accept her deep-sea identity with Otomi’s unwavering support. The group encounters a variety of other sea-creature characters, each introducing a lesson about marine biology integrated into a school comedy problem, such as a needlefish mermaid who cannot resist charging at light sources, a sunfish mermaid obsessed with sunbathing, and a great white shark mermaid whose scary reputation contradicts her gentle nature. The series balances educational facts about fish species with the gentle humor of high school friendships, showing how the characters overcome their insecurities and prejudices related to their distinct species.