On his first day of school, Katsuragi Musashi, known as the "Unsinkable" due to his undefeated status in fights, attends class a month into the semester. Shortly after his arrival, he finds himself in a brawl when he intervenes to protect his friend, Kaoru. Katsuragi proves his fighting ability by effortlessly taking down two opponents, but his situation worsens when six more attackers appear. Fortunately, Kagura Mami, the school's vice president and popular idol, intervenes and halts the altercation. This is unusual behavior for Kagura since she rarely assists others. During a game of tag with his childhood friend, Akane, Katsuragi stumbles upon an old school building with an enigmatic, large door that cannot be opened. The opening of this door would change his life and the lives of those around him. Kagura appoints Katsuragi as the new student council president, and living arrangements are made for him and several other chosen students. Unbeknownst to Katsuragi, these students possess animalistic supernatural powers. As if that weren't bizarre enough, Katsuragi discovers he now has a tail.
