"Mugen Shinshi: Bōken Katsugeki Hen" is a fantasy-adventure anime set in a world where humans coexist with supernatural beings. The protagonist, Mugen, is a wandering swordsman with a mysterious past, driven by a quest to uncover the truth about his origins and the enigmatic power he wields. His journey intertwines with a diverse cast of characters, each harboring their own motivations and secrets. Among them is Shizuku, a skilled warrior with a tragic past, and Rin, a spirited thief with a knack for survival. Together, they navigate a world filled with danger, intrigue, and ancient legends.

The narrative unfolds as Mugen and his companions are drawn into a conflict involving a powerful artifact known as the "Eternal Key," said to grant its wielder control over the boundaries between dimensions. Various factions, including a shadowy organization and rival adventurers, seek the key for their own purposes, leading to a series of battles, alliances, and betrayals. The story explores themes of identity, loyalty, and the consequences of wielding immense power, as the characters confront their pasts and the moral dilemmas posed by their quest.

Throughout the series, the protagonists face formidable adversaries, including rogue spirits, corrupt nobles, and ancient guardians, each encounter revealing more about the world's lore and the true nature of the Eternal Key. The plot is punctuated by moments of intense action, strategic planning, and emotional depth, as the characters' relationships evolve and their individual arcs progress. The anime balances episodic adventures with an overarching narrative, culminating in a climactic confrontation that tests the limits of their resolve and unity.