The series take place in the future, after mankind migrated to the planet Mars, home to the Martians.

Before long, the humans begin persecuting the Martians, and the two species form a mutual hatred of each other.

The action takes place Heden City, a frontier town and home to the Hoshino family.

Not long after she arrives, a mysterious gunman calling himself "Captain Ken" appears in Heden City.

Captain Ken is a human that fights on the side of the Martians, protecting them from those who wish to exploit them for their own gain.