Tetsuro Hoshino was once a boy willing to give anything to board the Three-Nine, including a promise to accompany a mysterious woman named Maetel to the other side of the Galaxy, if only to fulfill a vow to avenge the cruel death of his mother at the hands of the villainess Count Mecha. Now, two years after the events of Galaxy Express 999, Earth has become a battlefield, and Tetsuro is summoned to board the Three-Nine once more.