One summer day, a group of sixth-grade boys have an argument about whether fireworks are round or flat when viewed from different angles and embark on a journey for the answer during the annual firework festival. Meanwhile, one of their classmates, Nazuna, is troubled by her parents' separation and decides to choose one of the boys to run away with.

A year has passed since Menma's ghostly return to the Super Peace Busters. Although the time they spent together during that summer was short, the five members reminisce about what happened as they each write a letter to their lost friend.

Hiroko attends the memorial service of her fiancé, Itsuki Fujii, who died in a mountain-climbing incident. Although Itsuki's mother says that their old house is gone, Hiroko records the address listed under his name in his yearbook and sends him a letter. Surprisingly, she receives a reply, and discovers it came from his old classmate, a girl who also happens to be called Itsuki Fujii.

Moemi is not overly pleased when Yukio brings home a couple of turtles to keep her company. Although Yukio works at home as a writer, Moemi feels neglected and desired a dog or a cat, but neither is allowed in their apartment.

A dragon fish is stolen by Tetsu Tobiyama and biologist Thomas Earwing seeks its return. Poo is led to Natsuro after she worms her way into a top secret file at Delta headquarters for information.

Believing that the world will end that very day, three mental patients Coco, Tsumuji, and Satoru set out upon a journey. Walking upon the tops of the walls of the city, they seek to find a picnic spot with the best vantage point to view the final event.