"Undefeated Bahamut Chronicle" is a manga adaptation of a light novel series, set in a world where humanity relies on ancient weapons known as Drag-Rides to protect themselves from external threats. The story unfolds in the Academy City of Arcadia, a prestigious institution where students train to become Drag-Knights, skilled pilots of these powerful mechanical armaments. The Drag-Rides are remnants of a bygone era, and their operation is tied to the mythical Bahamut, a legendary dragon whose power is both revered and feared.

The protagonist, Lux Arcadia, is a former prince of the now-defunct Arcadia Empire. Once known as the strongest Drag-Knight, Lux was forced to betray his own family during a rebellion, earning him the nickname "Weakest Undefeated." Stripped of his royal status, he now lives as a commoner and works as a servant at the Academy. Despite his past, Lux possesses exceptional skill in piloting Drag-Rides, though he refuses to fight due to the guilt and trauma of his actions during the rebellion.

Lux's life takes a dramatic turn when he encounters Lisesharte Atismata, the princess of the new ruling kingdom and a student at the Academy. Lisesharte, or Lizsharte, is a proud and headstrong individual who initially views Lux with disdain due to his reputation. However, after witnessing his abilities firsthand, she coerces him into becoming her personal servant and partner in the Academy's Drag-Knight training. Their relationship evolves as they face various challenges together, including political intrigue, rivalries, and the resurgence of ancient threats tied to the Bahamut.

As the story progresses, Lux is drawn into a larger conflict involving the Academy's elite students, each of whom wields unique Drag-Rides and harbors their own ambitions. Among them are Philuffy Aingram, a childhood friend of Lux with a mysterious connection to the Bahamut, and Krulcifer Einfolk, a cunning and enigmatic girl who takes an interest in Lux's abilities. The group uncovers secrets about the Drag-Rides, the Bahamut, and the true nature of the world's power structure, leading to confrontations with powerful adversaries and ancient forces.

The narrative explores themes of redemption, loyalty, and the burden of power, as Lux grapples with his past and strives to protect those he cares about. The series blends action, fantasy, and elements of romance, with a focus on the relationships between the characters and their growth as individuals. The world-building incorporates a mix of futuristic technology and mythical lore, creating a setting where the past and present collide in unexpected ways. Through its intricate plot and dynamic cast, "Undefeated Bahamut Chronicle" presents a tale of resilience and the pursuit of a brighter future amidst the shadows of history.