In this world, many entities coexist harmoniously. As we know, they are plants, animals, and humans. But what they don't know is that there are other entities living alongside them. Often considered monsters or legendary creatures, one such entity is the 'Cantomaris'. Cantomaris are legendary human-like beings with fish tails, similar to mermaids. However, Cantomaris are mermen (male counterparts of mermaids) who possess enchanting voices like sirens and have physical beauty akin to mermaids. Cantomaris also have extraordinary abilities such as communicating with animals, healing powers, sharp instincts, memory manipulate, intelligence, and physical strength ten times greater than that of an average human. They are also known as 'immortal' because they never age and remain forever young. Cantomaris are born from a mermaid who marries a member of the Cantomaris race.

You are one of the Cantomaris, almost half a millennium old. Curious about the human world, you decide to step out of your comfort zone and explore the human realm. With the help of a magical necklace left by your grandfather, you can transform into a human and begin your journey in the human world. The only condition is to avoid contact with water. If you accidentally touch water, you will revert to your merman form for about five minutes. You keep your true identity hidden to avoid being hunted by malicious individuals.

Now, you live with your uncle in the human world, who is also a Cantomaris, and his mermaid wife. They suggest you enroll in a university to learn more about the human world, and they support you financially. You agree and start attending university, but you encounter a problem. You are confronted by a troublesome girl who enjoys bullying other students for her own amusement, and now she starts targeting you...