Along with three others, Natsu wakes up with no idea what's happening. The four of them reach an island and try to figure out a way to survive.

Convinced that Botan has kidnapped them, Natsu and Arashi sneak out of camp and run into another survivor. Finally, Botan reveals what she knows.

Natsu's clumsiness indirectly leads to Team Summer B to the rest of the group. Her memories help them figure out their location, and their next moves.

Arashi, Natsu and Semimaru stumble upon the Team Autumn settlement in the former Kobe region. They seem to be thriving, but their leaders are tyrants.

In the ruins of Tokyo, Natsu, Arashi and Semimaru meet the sole survivor of Team Winter. Team Spring reaches Mt. Ogino Fuji and finds Natsu's message.

Aramaki joins Team Spring, despite Fujiko's doubts. Hana decides to search for Arashi after hearing that Aramaki met someone who fits the description.

Hana and her companions discover a massive underground shelter. They find a survivor’s diary and learn the cruel fate of those who took refuge there.

Hana's group and the Team Autumn survivors find the Spring settlement destroyed and abandoned. Momotaro wakes up among strangers.

To survive, Hana and the others work together with Team Summer A. On the ship, Hotaru's cousin Hibari tells Natsu that Hotaru is slowly dying.

Ango recalls his Harsh upbringing in the program and the brutal graduation test that winnowed the survivors down to the final Team Summer A members.

Hana becomes convinced that Ango will kill her if she stays in the village. Natsu and the others make landfall near the former Autumn settlement.

Ryo orders Hana to join him in searching for a water source underground, putting her in a vulnerable position. In the caves, Ango recalls Shigeru.