Humans can no longer live on the contaminated surface of the planet, although there are magnificent landscapes, nobody would survive the impure air, although it is necessary to search for resources by incessant human survival underground.

Using beautiful and giant robots, called "guardians", the more skilled man warrants to search for supplies, or resources to defend the new country under attack from foreign countries too hungry for the resources needed for survival.

Junsu and Yoochun are piloting guardians to capture sunlight, water and soil nutrienets that can still be used.

Changmin and Jaejoong are piloting figthers guardians, they are soldiers willing to sacrifice their lives for the defense of the nation.

 Yunho pilots a rescue unit.

Junsu can not stand to see one who has always loved risking his live day after day, and Changmin don't care about the nation or whatever, he just wants to protect the one he trully loves. They can not play with love on the world that is shaken and the days of requiring continuing battle throughout the meaning of their lives, but what would that mean?