Two planets, one to the East and another to the West, were once bound together by a chain called the Dragon Stream. But now, that chain is broken and the two halves are only joined in war. Nobunaga the Fool is heir to the Eastern Country of Owari. Regarded as too foolish and carefree by many, including his friends, Nobunaga is thought to be a nuisance even by his father. A girl from the West named Jeanne Kaguya d'Arc has visions of a "Savior-King." She is accompanied by Leonardo da Vinci as she journeys to the Eastern Planet in search of the person in her visions. Leonardo and Jeanne quickly fall victim to a military confrontation between powerful mecha, only to be saved by Nobunaga. Nobunaga takes control of Leonardo's mecha, intending to warn his family of the siege he suspects. Jeanne suspects that the lackadaisical Nobunaga might be the Savior-King she's envisioned.

Sixteen years after its destruction by a meteor, Graviton City has been rebuilt into a futuristic metropolis. However, this is of little concern to superpowered high school student Eiko Magami, who only cares about her best friend Shiiko Kotobuki and arriving at school on time. Unfortunately, the incredibly intelligent Biko Daitokuji intends to befriend Shiiko, taking Eiko's place as the Shiiko's best friend using any means she can. Biko decides that, to accomplish this, she must defeat Eiko in combat. In order to overcome her super strength and speed, Biko creates a mecha and challenges Eiko in front of their school gate. As the two commence their battle, an extremely large spaceship hovers over Earth, searching for something left behind. [Written by MAL Rewrite]

Three weeks after the incident that left Captain Napolipolita's ship balancing on top of Graviton City, A-Ko and the gang go on summer vacation. While A-Ko ponders losing some weight and B-Ko devises another plan to defeat her redheaded rival to win C-Ko, Napolipolita and Spy-D experience severe homesickness - begging for a way to return to their home planet. Meanwhile, Hikari Daitokuji - B-Ko's father and the CEO of the Daitokuji Financial Group - arms the local military with new mecha to attack Napolipolita's ship and obtain its advanced technology. (Source: ANN)

In Iraq, a group of archaeologists discover ancient relics that prophecize the coming of a superior race and the end of all civilization. Meanwhile, back in Graviton City, as A-Ko and B-Ko's rivalry over Kei intensifies, Mr. Daitokuji sets up an arranged marriage between Miss Ayumi and Kei. Furious over the engagement, both A-Ko and B-Ko do whatever it takes to prevent the wedding from going through. Little do they know that another alien fleet is on its way towards Earth. (Source: ANN)