In a future where eternal life comes with a hefty price tag of mechanical bodies, a destitute youth named Tetsurou Hoshino dreams of boarding the Galaxy Express 999, a cosmic train promising a free upgrade at its final stop. Tragedy strikes when his mother falls to the sinister Count Mecha, a malevolent machine-being. Just as hope fades, Tetsurou is rescued by the enigmatic Maetel, a woman resembling his late mother. Entrusted with a ticket to the Galaxy Express, Tetsurou's sole condition is to journey alongside Maetel. Thus begins his odyssey through space, braving diverse worlds and daring escapades in pursuit of his ultimate goal.