Aired with the rebroadcast of Isekai Cheat Magician. Ordinary high school students Taichi Nishimura and Rin Azuma are walking to school one morning when they are suddenly transported to a magical world.

Waking up in a new world, 25-year-old Shingo Ichinomiya realizes that he is in the body of a six-year-old. Retaining memories of his stressful life working at a firm company, Shingo learns that the person he is occupying is Wendelin Von Benno Baumeister, the eighth son of a poor noble family living in the countryside. Awoken to his bizarre situation, Wendelin strives to change his financial and social status for the better. His newly discovered great magical aptitude may prove to be just what he needs to achieve that goal.

Tanya Degurechaff is a soldier who has the appearance of an innocent young girl. But behind her cute facade lies the soul of a ruthless salaryman who challenged "Being X"—the self-proclaimed God who reincarnated the man into an industrial world of magic. By doing this, Being X hopes to prove that, if the situation were dire enough, even a non-believer like this man would pray for God's help. Opposing Being X's desires to make her suffer, Tanya enlists in the military with the hope of living a peaceful life. She manages to impress the higher-ups with her aptitude for war, swiftly rising through the ranks. However, her country soon participates in the ongoing world war, quickly crushing her dream. In an attempt to return to her calm she starts using any merciless means to obliterate her opponents. Thus begins the saga of Tanya the Evil and the Empire as they seek world domination.

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?