The Free Planets Alliance Navy Expeditionary Fleet is fighting a severe supply shortage, and the supply convoy that was sent to save them has been destroyed by Kircheis in a fierce attack. Without their desperately needed supplies, the disheartened Alliance fleet takes a hard hit from the Imperial Navy. As Reinhard's subordinates take out the Alliance fleets one by one, Yang's 13th Fleet is attacked by the Kempff Fleet.

Emperor Friedrich IV of the Galactic Empire has suddenly passed away without naming a successor. Edwin Josef, the son of the late prince and the emperor's only surviving direct descendant, is only five years old. The Braunschweig and Littenheim families, into which the emperor's daughters had married, begin scheming to install their respective daughters as the new empress. Thus begins the battle for the throne between the families who serve as guardians to the emperor's three grandchildren. Reinhard has not sided with any of them yet, so a certain influential power seeks to take advantage of his military strength.

Animosity continues to grow between Reinhard and Lichtenlade, the guardians of the young emperor Erwin, and the pure-blooded noble families with Braunschweig and Littenheim at their center. In the midst of all this, Hilda, the only daughter of Count Mariendorf, visits the Fleet Admiral's Mansion on Odin to meet with Reinhard. When he asks what brought on the sudden visit, she tells him that the Mariendorf household would like to side with Reinhard in the civil war that could break out at any moment.

Only a short time has passed since the Alliance's failed invasion of the Empire, and an assassination attempt on Admiral Cubersly, a key figure in the Alliance Navy, further deepens the people's anxiety. On top of that, an accidental explosion the following day takes fourteen young soldiers' lives. Growing discontent has caused revolts to break out on several planets within the Alliance. Bewcock orders Yang to put down these revolts, but before Yang has a chance to deploy, a coup d'état is executed on the capital planet Heinessen.

The Lippstadt Nobles Coalition devises a plan to lure Reinhard's forces away from the Imperial capital, Odin, and capture the emperor while Imperial defenses are spread thin. While the nobles fight among themselves over who gets to lead this strike, the Mittermeyer Fleet lays a minefield that forces the rebels off-course, and then strikes them from behind. The defeated rebels flee to Rentenberg Fortress. Reinhard sends Reuenthal and Mittermeyer to occupy the fortress, but upon arriving, they are forced into close-quarters combat with the notoriously barbaric High Admiral Ovlesser.

After easily putting down a revolt on the planet Shampool, Yang is approached by Commander Baghdash, a Department of Intelligence official. Baghdash informs Yang that the 11th Fleet has joined the coup d'état and is headed his way. While the Yang Fleet and the 11th Fleet engage in the Battle of the Doria Starzone, the coup d'état faction remains steadfast, intervening with military force at a peaceful protest on the capital planet Heinessen.

After defeating Legrange at the Doria Starzone, Yang's fleet heads to the capital planet Heinessen, which has been taken over by the National Salvation Military Council. Though the council's chairman, Greenhill, understands the severity of the situation, he still believes there's no way Yang's fleet can even penetrate Heinessen's gravitational field because of its specialized defense system, Artemis's Necklace. But Yang had a plan to disable Artemis's Necklace...