Friends become enemies as the OSF splits in two. Yuito receives a promotion, but why can't he remember? Karen creates a new Seiran OSF platoon and tells Kasane they have Naomi in their care. The hunt for Nagi is on as Yuito gains fresh intel.

Kagero provides more info on the Ark mission and explains the Extinction Belt's origin. A new threat stalks the platoons, but their escape route is even more dangerous. They must decide their next step once Togetsu's ultimate objective is revealed.

A shock awaits Yuito and company when they confront Karen in the mausoleum. Can they convince him to help? Yuito and Kasane deal with their regrets over what happened to Nagi and Naomi, and they must take drastic measures in an attempt to save the world.