When 50-year-old Kenichi and his childhood friends reunite at the funeral of their classmate, they decide to bring a long-forgotten dream back to life—the "secret base construction plan" they'd drawn up as kids. They have just six days that summer to build their hideout without cars, electric tools or smartphones. Rediscovering the spirit of their youth, the men throw themselves into the challenge with all their hearts. Still, they are unable to escape the struggles they face as adults.