"Phoenix: Eden17" is a science fiction anime series set in a distant future where humanity has colonized various planets in search of a new home. The story centers on Romi and her younger brother, Yuu, who are part of a group of colonists aboard the spaceship Eden17. After a catastrophic event renders their ship uninhabitable, the siblings are forced to evacuate and land on a mysterious, seemingly lifeless planet. The planet, initially barren and hostile, begins to reveal strange phenomena, including the presence of a mythical creature known as the Phoenix, which is said to possess the power to grant eternal life.

As Romi and Yuu explore the planet, they encounter remnants of an ancient civilization and uncover clues about the Phoenix's existence. The siblings face numerous challenges, including environmental hazards, dwindling resources, and the psychological toll of isolation. Their journey is further complicated by the arrival of other survivors from Eden17, each with their own agendas and secrets. Tensions rise as conflicting motivations and the struggle for survival test the bonds between the colonists.

The narrative delves into themes of hope, resilience, and the human desire for immortality, as the characters grapple with the ethical implications of seeking the Phoenix's power. Romi emerges as a determined and resourceful leader, driven by her desire to protect her brother and ensure the survival of their group. Yuu, on the other hand, represents innocence and curiosity, often serving as a moral compass amidst the group's growing desperation.

The plot unfolds through a series of discoveries and confrontations, both external and internal, as the colonists navigate the planet's mysteries and their own fractured relationships. The Phoenix, a symbol of rebirth and transcendence, becomes a central focus, with its elusive nature fueling both hope and conflict. The series builds toward a climactic exploration of the planet's core, where the truth about the Phoenix and the fate of humanity's quest for immortality is ultimately revealed.