Train Heartnet is an amicable bounty hunter—known as a "Sweeper"—with a talent for sharpshooting. Working alongside his cool-headed partner Sven Vollfied, they hunt down wanted criminals. Unfortunately, however, despite their efforts, the dynamic duo always seems to be short on money, which forces them to accept a job from the beautiful Rinslet Walker, a sly thief for hire, and a mysterious girl named Eve. However, the motley crew soon discovers a terrible plot to revolutionize the world, and the mastermind is someone Train knows and loathes with a passion. As the plan is set into motion, it is soon made clear that behind the carefree sharpshooter's smiling face lies a tragic past and an insatiable desire for revenge.