From MangaHelpers:
The island nation of Tsushima is comprised of 3 different races of people. The Akitsu race is the majority and comprises of 80% of the population, the Shiomi population comprises of the upper 10% and the Oina race, most known for their tail and neko mimi (cat ears), makes up the remaining small percentage. Tsushima is also known for its relatively strong military force.
Mikasa Sanekiyo, formerly of the Sakune family, was adopted by a family after his parents and grandparents passed away. His older brother-in-law, Mitsukiyo, is already an accomplished military commander and Sanekiyo looks up dearly to him for moral and emotional guidance. Following in his footsteps, Sanekiyo is accepted into the navy academy as the top chair of his class. Little does he know how discriminatory the Akitsu are against the other two races and how hard the training is going to be. Sanekiyo has to learn how to be a great leader and overcome these obstacles if he plans to lead the navy fleet someday.