From J-Wiki:
Takashi is reunited with a childhood friend he has not seen in 13 years named Miriko. Miriko, while thought to be missing, became the queen of an alien planet and leader of a strange army corp. To keep a childhood promise, Miriko sets out to take over the world with Takashi. However, Takashi has no desire to do this, so Takashi must stop Miriko's wild behaviors.