"Brigadoon" is a 26-episode anime series that blends science fiction, fantasy, and drama, set in a unique world where the titular floating city of Brigadoon periodically appears above Earth. The story centers on Marin Asagi, a young orphaned girl living in 1960s Japan, whose life takes a dramatic turn when Brigadoon descends and targets her for unknown reasons. The city is inhabited by advanced beings known as Monomakia, who possess powerful weapons and technology. Marin becomes the target of assassins from Brigadoon, forcing her into a desperate struggle for survival.

To protect herself, Marin forms a bond with Melan Blue, a sentient weapon known as a "Protector," who emerges from Brigadoon to defend her. Melan Blue is a stoic and enigmatic figure, driven by a sense of duty to safeguard Marin. As the series progresses, it becomes clear that Marin's connection to Brigadoon is deeper than it initially appears, tied to a mysterious prophecy and her own hidden past. The narrative explores themes of identity, sacrifice, and the consequences of wielding power, as Marin and Melan Blue navigate a world filled with danger and intrigue.

The antagonists, known as the "Monomakia," are a diverse group of individuals with their own motivations and backstories, often clashing with Marin and Melan Blue in intense battles. The series delves into the complexities of their society, revealing political tensions and moral dilemmas within Brigadoon. As Marin uncovers the truth about her role in the prophecy, she must confront her fears and make difficult choices that will shape the fate of both Earth and Brigadoon.

The supporting cast includes Lolo, Marin's younger sister figure, and other allies who provide emotional depth and context to the story. The series balances action-packed sequences with moments of introspection, exploring the relationships between characters and the broader implications of their actions. The narrative builds toward a climactic resolution, where Marin's decisions ultimately determine the future of both worlds. "Brigadoon" is a compelling blend of personal drama and larger-than-life stakes, offering a thought-provoking exploration of its central themes.