The start of Hanae Ashiya's high school career has not been easy—he has spent all of the first week in the infirmary, and his inexplicable condition is only getting worse. The cause of his torment is the mysterious fuzzy creature that has attached itself to him ever since he stumbled upon it the day before school began. As his health continues to decline and the creature grows in size, Hanae comes across a flyer advertising an exorcist who expels youkai. Desperate and with nothing left to lose, he calls the number and is led to the Mononokean, a tea room which suddenly appears next to the infirmary. A morose-sounding man, Haruitsuki Abeno, reluctantly helps Hanae but demands payment afterward. Much to Hanae's dismay, he cannot afford the fee and must become an employee at the Mononokean to work off his debt. And to make things worse, his new boss is actually one of his classmates. If Hanae ever hopes to settle his debt, he must work together with Abeno to guide a variety of dangerous, strange, and interesting youkai back to the Underworld.

Led by Reinhard von Müsel, the Galactic Empire crushes the Free Planets Alliance in the Fourth Battle of Tiamat. Arriving in the Imperial capital of Odin to a hero's welcome, Reinhard's victory earns him a promotion to High Admiral and the distinguished title of Count von Lohengramm. On the other side, Yang Wen-li safely returns to the Alliance capital of Heinessen without fanfare and reunites with his dear friends. Reinhard and Yang look forward to a long-awaited reprieve. However, at the request of the scheming Duke Otho von Braunschweig, the Imperial High Command orders the new High Admiral to invade the Alliance with a fleet of twenty thousand ships. After the Duke leaks the plan to the Alliance government, three fleets totalling forty thousand ships are sent to counter the incoming attack and recreate the famous Battle of Dagon. But even though Reinhard is outnumbered two to one, he has an ingenious plan—and Yang sees right through it. Will Yang's superiors heed his warning before it is too late?

Four months before Count von Lohengramm of the Galactic Empire faced Yang Wen-li of the Free Planets Alliance at the Battle of Astarte, he was still just Reinhard von Müsel. The youngest admiral in the Empire's history, Reinhard was disdained and dismissed by his peers as the brother of the Kaiser's concubine. Upon arriving at Iserlohn Fortress with his expeditionary fleet, Reinhard immediately receives an order from the ambitious and cunning Fleet Admiral Gregor von Mückenberger: to intercept an Alliance fleet in a neighboring starzone. Despite recognizing this as a veiled attempt to get him killed in combat, Reinhard nevertheless orders his tired and weary men to engage the enemy in the atmosphere of the gas giant Legnica. But unbeknownst to him, this will mark the first of his many historic encounters with the Hero of El Facil.

The 150-year-long stalemate between the two interstellar superpowers, the Galactic Empire and the Free Planets Alliance, comes to an end when a new generation of leaders arises: the idealistic military genius Reinhard von Lohengramm, and the FPA's reserved historian, Yang Wenli. While Reinhard climbs the ranks of the Empire with the aid of his childhood friend, Siegfried Kircheis, he must fight not only the war, but also the remnants of the crumbling Goldenbaum Dynasty in order to free his sister from the Kaiser and unify humanity under one genuine ruler. Meanwhile, on the other side of the galaxy, Yang—a strong supporter of democratic ideals—has to stand firm in his beliefs, despite the struggles of the FPA, and show his pupil, Julian Mintz, that autocracy is not the solution. As ideologies clash amidst the war's many casualties, the two strategic masterminds must ask themselves what the real reason behind their battle is.