Space Symphony Maetel: Galaxy Express 999 Side Story(宇宙交響詩メーテル ～銀河鉄道999外伝～,Supēsu Shinfonī Mēteru: Ginga Tetsudō Surī Nain Gaiden?) is a 2004 anime series created by Leiji Matsumoto, and is a side story to the Galaxy Express 999 series and the sequel to Maetel Legend.
Maetel had no choice but to either run away from her home on La Metal or stay and take over her mother's job of ruling over it. After being asked to take over for her mother, Maetel is forced to return to La Metal. On the way there, she meets a young boy named Nasuka who is traveling to La Metal for a different reason; Revenge. On arriving she quickly comes to realize her mother has been forcing all the humans on the planet to gain mechanical bodies as her planet is being over come with another ice age.
The mechanized beings then slowly move to other galaxies, turning others into cyborgs against their will. Then rebels stood up against the beings and a war followed out. Maetel, now thrown into the middle of the battle, is now stuck between what is right and what is duty.