In the kingdom of Earlshide, Merlin Wolford was once regarded as a national hero, hailed for both his power and achievements. Preferring a quiet life however, he secludes himself deep in the rural woods, dedicating his time to raising an orphan that he saved. This orphan is Shin, a normal salaryman in modern-day Japan who was reincarnated into Merlin's world while still retaining his past memories. As the years pass, Shin displays unparalleled talent in both magic casting and martial arts, much to Merlin's constant amazement. On his 15th birthday, however, it becomes apparent that Shin only developed his combat skills and nothing else, leaving him with blatant social awkwardness, a lack of common sense, and a middling sense of responsibility. As a result, Shin enrolls in the kingdom's Magic Academy to hone his skills and mature among other teenagers. However, living a normal life is impossible, as he is established as a local celebrity almost as soon as he arrives. As Shin Wolford adjusts to his high school life in the capital, he makes new friends, learns about the world, and fights off the various forces of evil surrounding him and his city.

Following the conclusion of the three-way organizational war, government bureaucrat Ango Sakaguchi recalls an event that transpired years ago, after the death of the former Port Mafia boss. Osamu Dazai, still a new recruit at the time, was tasked with investigating rumors related to a mysterious explosion that decimated part of the city years ago—and its connection to the alleged reappearance of the former boss. Due to circumstances out of his control, he is partnered with Chuuya Nakahara, the gifted yet impulsive leader of a rival clan known as the Sheep, to uncover the truth behind the case and shine a light on the myth of Arahabaki—the god of fire who might just lead Dazai to the case's solution. Meanwhile, in the present day, it is business as usual once again for the Armed Detective Agency. Their peaceful break will not last for long, however, as enemies old and new gather their strength and prepare for another face-off.

After being trapped in the world of Elder Tale for six months, Shiroe and the other Adventurers have begun to get the hang of things in their new environment. The Adventurers are starting to gain the trust of the People of the Land, and Akiba has flourished thanks to the law and order established by Shiroe's Round Table Alliance, regaining its everyday liveliness. Despite this success, however, the Alliance faces a new crisis: they are running out of funds to govern Akiba, and spies from the Minami district have infiltrated the city. As formidable forces rise in other districts, there is also a need to discover more about the vast new world they are trapped in—leading Shiroe to decide that the time has come to venture outside the city. Accompanied by his friend Naotsugu and the Sage of Mirror Lake Regan, the calculative Shiroe makes his move, hoping to unravel new possibilities and eventually find a way home.

An ancient legend states that with the revival of the Demon God, six heroes—the Braves of the Six Flowers—will be chosen by the Goddess of Fate, granting them power to rise up against the fiends attempting to turn the world into a living hell. Adlet Mayer, self-proclaimed Strongest Man in the World, has arrived at the continent of Piena in hopes of becoming a Brave. Although it doesn't go as smoothly as he had planned, Adlet is ultimately chosen as one of the six heroes shortly after being greeted by Nashetania Loei Piena Augustra, crown princess and fellow Brave. Rokka no Yuusha follows the two as they embark upon their destined journey to fight the Demon God, intending to meet up with their fellow heroes at a small temple outside of the Land of the Howling Demons, the fiends' domain. However, when they finally unite, seven heroes are present, and soon the others begin to suspect Adlet to be a fraud. Now on the run, Adlet must utilize his unique skill set and wit in a fight for his life to identify which member of the group is the true impostor before it's too late!