Setsu Suzaki and his classmates are suddenly summoned to a foreign world, and arrive at a kingdom that is just about to plunge itself into war. Most of the children are confused, except for Setsu, because this isn't his first time.

Setsu sets off for the demonic continent - and into a chance meeting with the granddaughter of a certain merchant he met once in his previous life, who has her own duty to fulfill...

Setsu, Ruri, and Levia arrive on the demonic continent. They reconnect with old acquaintances and are swept up in a sinister plot involving the Demon Lord and a mysterious merchant.

The Demon Lord Dezastol reflects on the previous war and the promise of peace she made to Setsu. Meanwhile, Setsu, Levia, and Vlad rush to rescue her from the unwanted marriage to the mysterious Terran Sneeter.

While Setsu is fighting his battles on the demonic continent, Yuhi trains to become strong enough to stand by his side. With the help of Elka's tough love, she discovers her own unique kind of determination.

Setsu and Levia set off for the Beastmen continent. On their way, they cross paths with Setsu's old friend Alizee and discover she has been battling a mysterious dragon. Together, they unravel the shocking truth.