In the distant future, people are able to take on artificial bodies and achieve immortality. A poor orphan, Tetsuro Hoshino, strives to obtain a mechanical body for the triple incentive of eternal life, of revenge (his mother was gunned down by machine men) and to fulfill her dying wish (that he acquire a mechanical body to live forever, and not die like her). He plans to board the Galaxy Express 999, a space train that comes to Earth once a year, and head to the Prometheum, a planet that gives out machine bodies for free. To his surprise, Maetel, a mysterious woman who resembles his mother, selects him as her companion aboard the 999. And thus begins Tetsuro's trip through the stars, where he encounters incredible worlds and makes many friends. But as he travels with Maetel, he witnesses the hardships of mechanical life and wonders, is it worth it? And what secret is the inscrutable Maetel hiding?