The year is 2053 AD. The Earth has lost a huge portion of its population to a mysterious life form that has appeared from space. With all of mankind's attempts to counterattack ending in failure, the Fuji Base Japanese Area Forces puts their final plan to strike the enemy location into action; Operation Planzet. Hiroshi Akishima, pilot of the robot weapon, has been ordered to protect the people at Fuji Base without fail. Before the battle Hiroshi's little sister Koyomi, his only surviving family, takes refuge on the safe planet of Mars. Thus begins Planzet, the battle for mankind's fate.