Following mankind's arrival on Mars, the Martians and humans develop a fierce animosity towards each other, resulting in persecution of the Martians by humans.
Residing in the frontier town of Heden City, the Hoshino family receives a visit from their relative, Ken Minakami. Shortly after her arrival, a gunslinger known as "Captain Ken" emerges in the town and aligns himself with the Martians, shielding them from exploitation.
As Mamoru Hoshino, the eldest son of the family, observes the events unfolding, he begins to suspect that his relative, Ken Minakami, may be the elusive Captain Ken.
