"Katsugeki! Tōken Ranbu" is set in an alternate version of Japan during the late Edo period, where historical swords are personified as warriors known as Touken Danshi. These warriors are tasked with protecting history from the Time Retrograde Army, a group of malevolent forces seeking to alter the past. The story primarily revolves around two units of Touken Danshi: the First Unit, led by Izuminokami Kanesada, and the Second Unit, led by Yamanbagiri Kunihiro. Each warrior embodies the spirit and traits of their respective swords, with their personalities and abilities reflecting their historical origins.

The narrative begins with the First Unit being dispatched to investigate disturbances in the timeline, particularly during the tumultuous Bakumatsu period. The warriors encounter the Time Retrograde Army, which aims to disrupt key historical events, such as the burning of Kyoto. As the story progresses, the Touken Danshi confront the moral complexities of their mission, questioning whether their actions to preserve history might inadvertently cause further harm. The series delves into themes of duty, loyalty, and the weight of history, as the characters grapple with their roles as protectors of the past.

Key characters include Izuminokami Kanesada, a passionate and headstrong leader who wields the spirit of a sword forged by the renowned swordsmith Kanesada. His close companion, Horikawa Kunihiro, serves as a calming influence and provides strategic insight. Yamanbagiri Kunihiro, the leader of the Second Unit, struggles with his identity as a replica sword but demonstrates unwavering resolve in battle. Other notable characters include Mutsunokami Yoshiyuki, a cheerful and optimistic warrior with ties to the Shinsengumi, and Tsurumaru Kuninaga, a mysterious and enigmatic figure with a penchant for unpredictability.

The plot intertwines historical events with the Touken Danshi's missions, blending action, drama, and moments of introspection. The series explores the emotional toll of their eternal duty, as the warriors must repeatedly face the same battles across different timelines. The Time Retrograde Army serves as a persistent threat, with their motives and origins gradually revealed as the story unfolds. The narrative culminates in a climactic confrontation that tests the resolve and unity of the Touken Danshi, as they strive to safeguard the integrity of history while confronting their own inner conflicts.