21st Century Boys Vol 1

From the VIZ Media edition

War is over. The Friend is dead. Mankind no longer faces the threat of extinction. Peace has finally come to Tokyo... or has it? The mystery still remains. Nobody knows who the Friend was and where he came from. The only clue is hidden deep within the memories - the memories of the hero Kenji. It is time to open Pandora's Box to discover what is left at the bottom.

Source: Anime News Network Encyclopedia