The older boys are graduating school and recount their memories together. The next day everyone defends the graduation ceremony against Zundar and Dadacha's Second Button Monster but Yumoto struggles using his love power.

In the kingdom of Earlshide, Merlin Walford 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. Kenja no Mago follows Shin Walford's high school life in the capital as he makes new friends, learns about the world, and fights off the various forces of evil surrounding him and his city.

The day is as normal as it can be in high school as the students peacefully go about their everyday activities until an unprecedented catastrophe strikes the school, killing every person in its wake. Guided by what seems to be a miracle, a handful of students are fortunate enough to be reincarnated into another world as nobles, princes, and other kinds of people with prestigious backgrounds. One girl, however, is not so lucky. Being reborn as a spider of the weakest kind, she immediately experiences the hardships of her dire situation. Even so, she must press on to survive the numerous threats that endanger her life. Discovering that her new world has a system like that of an RPG, she tries her best to hunt prey and defeat monsters to level up and evolve. As she gradually grows stronger, she hopes one day her efforts will be rewarded, and that she will be granted a better life.

With its armies sweeping across the continent, the Empire seems unstoppable. After securing victory over the remnants of the Republic's army, the Empire's ultimate victory is finally within reach. However, dark clouds are gathering in the East. The communist-led Russy Federation is mustering troops on its western border, preparing to enter the war. Supported by a detachment of Allied volunteer magicians—among whom is Mary Sioux, the daughter of a soldier killed by Tanya Degurechaff—the Federation is determined to spread the communist creed and bring the Empire to its knees.

Meanwhile, Tanya and her battalion return to the imperial capital from the southern front. Upon their arrival, they are tasked with investigating troop movements on the border with the Federation. Any escalation of violence at this point may lead to new conflicts, plunging the world into a devastating global war.

Will the Empire eventually emerge victorious from its struggle, or will it crumble in the face of superior enemies and radically different ideologies?