In 1945, a war-weary world is beset by an alien invasion of supernatural origin. Humanity is united by the common threat, but the alien onslaught decimates the world's already weakened military forces. By 1999, Japan has no choice but to turn to its teenage population to carry on the fight. This is the story of Unit 5121, a group of high school students who must balance homework and battle in the Human Walking Tanks.