An alien force, known as “Phantom beasts,” appeared on earth during the middle of War World II causing an abrupt end. 54 years later mankind is still fighting the alien force using mecha known as HWTs, Humanoid Waking Tanks. Despite the advances in technology the numbers of those killed has caused a deficit in military soldiers. As a result the Japanese government enforced a student draft that lowed the draft age to 16. The 5121st Platoon consists of mostly high school students that have been drafted into the war. After the death of a member of a platoon, Mai Shibamura joins the group. Mai is a perfectionist but has difficulties with social skills. She is partnered up with Atsushi Hayami who is referred to as a goof and mediocre in almost all aspects. The two must work together to pilot the two manned HWT called Tandem.