In the middle of the 21st century, Tokyo's criminal underworld is dominated by mafia bosses who have undergone ether enhancement surgery, transforming them into monstrous beings known as Demon Kings. To counter this threat, the government sanctions bounty hunters called Braves, who use a powerful and dangerous drug known as E3 (E-Three) to boost their own ether abilities. The act of killing Demon Kings has been legalized, turning the title of Brave into a professional license for legalized murder rather than a mark of honor.

The story centers on Yashiro, a highly skilled but cynical freelance Brave known as the Grim Reaper. Disillusioned and perpetually broke, he prefers a life of simple pleasures like beer, pizza, and card games over heroics. His solitary existence is upended when a high school girl named Aki Jogamine forcibly barges into his life and declares herself his disciple. Aki is a student at the Brave Training School, an academy that turns teenagers into assassins, and she is driven by a strong, if naive, sense of justice. Reluctantly, Yashiro agrees to take her on, not out of altruism, but for a substantial sum of money offered by Aki and her two classmates, the stoic Yukine Indo and the delinquent-yet-refined Saera Kashiwagi Pendragon, who is the daughter of Arthur Pendragon, the head of the Round Table Foundation that oversees Braves worldwide.

The series follows the scummy master and his self-proclaimed disciples as they navigate Tokyo's violent underworld. Yashiro's pragmatic and often ruthless methods clash with Aki's idealistic views, creating a tense mentor-student dynamic. Their early missions together involve taking down minor Demon Lords for training, such as the Count of Amber Thorn, a regenerating foe whose power is eventually overcome by Aki's unique mind-reading ability. As Yashiro gets dragged into one absurd and dangerous incident after another, he also contends with a cast of fellow Braves, including his impulsive friend Salt Joe, the unnervingly cheerful Music Shop Ishinoo, and the socially awkward Diver Maruta.

Notable narrative arcs include Yashiro's reluctant agreement to train the three academy girls, which draws him out of isolation and into conflicts with more formidable enemies from his past. One such enemy is the Wedge of Both Hands, a formidable Demon Lord who recognizes Yashiro. The story also explores the internal politics of the Brave system, as seen when the girls' missions involve shutting down drug rings in Shinjuku nightclubs and dealing with the consequences of the E3 drug trade. A significant subplot revolves around Saera's attempts to earn Yashiro's respect using her family's influence, only to learn that trust cannot be bought, while Aki's unwavering dedication increasingly puts her in harm's way, forcing Yashiro to confront his own buried sense of responsibility. As the series progresses, Yashiro is drawn into escalating confrontations that place him at the center of a chaotic conflict between the Braves, rival criminal organizations, and the very system that created them.