Air and sea transportation to Itogami Island have been cut off entirely. Meanwhile, key personnel of the Gigafloat Management Corporation are being assassinated one after another. The mastermind behind this series of incidents is the self-proclaimed Demon Sanctuary Destruction group, Tartarus Lapus.

Nagisa is still suffering spiritual fatigue due to the awakening of the soul of Avrora. At this time, she meets a strange girl who goes by the name December. Unbeknownst to Nagisa, this girl is the leader of Tartarus Lapus, and she has a special ability capable of overwhelming even the Fourth Primogenitor!