A Japanese assassin, The Strongest warrior of the Unified Irish kingdom - Satoshi Kamado, was assigned to train the cadets from the military instituition Dullahan, as monsters' attacks get more violent, coordinated and deadly each year. The humanity needs a hope, and his ellite squad must become that hope. The only difficulty is that his comrades keep dying. His step-sister and his lover - assassin Kali Carsvell - is the only person in the world, who willingly protects him, when everyone rely on his protection. Dullahan is an Irish monster that is called a headless rider on a black horse, who carries his own head. The students of this instituition are self-sacrificial warriors, classified into knights and assassins, who are destined to die for humanity. Satoshi and Kali are not the only ones, who refuse to lose their loved ones. Older warriors are desperate to save the students of Dullahan from the hideous fate of being killed by the monsters. The new generation's survival is the prospect that Satoshi sees and strives to impose in Ireland. Along with being the humanity's Strongest protector, Sensei and Kamado clan's heir, his tormenting thing to deal with is his forbidden love for his step-sister. Kali knows nothing about her past, before she was adopted by Kamado family. Since then, Satoshi has been the center of her universe and the source of her grief. She sees him as a tool in humanity's hands, guiding him to death everytime he protects comrades on the battlefield. Her only wish is to be able to provide him a backup. Those star-crossed lovers are destined to seek their way out of sufferings, monsters' attacks, shackles of society's rejection and endless duties of being warriors. The fuel added to the fire of combat is the encounters of Knights and Assassins that fill the days in Dullahan with the spirit of competition, love and hard-work.