Since 1973, an invasion of aliens known as BETA upon Earth is driving human civilization into nearly collapse. Facing an enormous mass of enemy force, mankind has developed large humanoid arms called as Tactical Surface Fighters and deployed them to its defense lines through out the world. However, all those efforts could slow down defeats but could not stop BETA, and mankind is forced to abandon major area of the Eurasian Continent. For 30 years, mankind is bogged down in an endless war against BETA without any hopes of victory.
