Soon after mankind migrates to Mars, humans begin persecuting the Martians, and the two species form a mutual hatred of each other.

The Hoshino family lives in th frontier town of Heden City. One day their relative Ken Minakami comes to visit. Not long after she arrives, a mysterious gunman calling himself "Captain Ken" appears in Heden City. Captain Ken is a human fights on the side of the Martians, protecting them from those who wish to exploit the them for their own gain.

Mamoru Hoshino, the eldest son of the Hoshino family, begins to suspect that Ken Minakami may be the mysterious gunslinger, Captain Ken.