"Space Symphony Maetel ~Ginga Tetsudō 999 Gaiden~" is a two-episode OVA that serves as a spin-off to the iconic "Galaxy Express 999" series, exploring the backstory of Maetel, one of its central characters. The narrative is set in the distant future, where humanity has colonized the Andromeda Galaxy. The story delves into the origins of Maetel and her connection to the planet La Metal, a world ruled by the tyrannical Queen Promethium. La Metal is a mechanized society where humans are forced to abandon their organic bodies and transfer their consciousness into mechanical forms, a process enforced to maintain control and suppress rebellion.

Maetel, the daughter of Queen Promethium, is initially portrayed as a loyal subject to her mother's regime. However, her perspective begins to shift as she witnesses the suffering and oppression inflicted upon the people of La Metal. The story follows her internal conflict and growing disillusionment with her mother's rule. Maetel's journey is intertwined with that of Emeraldas, a fierce and independent space pirate who opposes Queen Promethium's tyranny. Emeraldas serves as a catalyst for Maetel's awakening, challenging her to question the morality of her mother's actions and the fate of their civilization.

The narrative explores themes of rebellion, identity, and the consequences of technological advancement. Maetel's transformation from a passive observer to an active participant in the struggle against her mother's rule is central to the plot. The OVA also provides insight into the broader "Galaxy Express 999" universe, offering context for Maetel's motivations and her eventual role in the main series. The story is driven by its exploration of power dynamics, the ethical implications of mechanization, and the enduring human desire for freedom and self-determination. Through its concise yet impactful storytelling, "Space Symphony Maetel" enriches the lore of the "Galaxy Express 999" saga while standing as a compelling narrative in its own right.