After losing his father and his brother, both members of the SDF (the Space Defense Force), Manabu decides to join up himself. On the way, he meets up with Louis Fort Drake, another new member of the SDF.

He's initially hated by Bruce for his pacifist attitudes and blithe refusal to carry a gun (other than the one he inherited from his father, which also doesn't work), and, like most youngsters, he's a bit of a hothead. Through the various missions undertaken, some of them more peculiar than others, he eventually settles into the role as an SDF member.