The name Taro Sakamoto was once legendary in the underworld. As the world's greatest hitman, his prowess was unmatched, and he was feared by villains and revered by fellow assassins. That life came to an abrupt end when he fell in love with a convenience store clerk named Aoi. Choosing family over violence, Sakamoto retired, got married, became a father, and put on a significant amount of weight. He now lives a quiet, peaceful life as the owner of a small neighborhood convenience store in Tokyo, having sworn a solemn oath to his wife that he will never kill again.

However, a peaceful life is not easily earned in the world of professional killers. After years of searching, his former associates finally locate him. The first to arrive is Shin Asakura, a young man with telepathic abilities who was once Sakamoto’s protégé. Shin is sent to eliminate his former mentor, but he is quickly and effortlessly defeated. Impressed by Sakamoto’s skills and touched by the warmth of his new family, Shin abandons his mission and becomes the first employee of the Sakamoto family store.

The situation escalates dramatically when a massive bounty is placed on Sakamoto’s head, turning his quiet shop into a battlefield. A relentless stream of assassins, old rivals, and eccentric killers descend upon his family, all hoping to claim the reward. To protect his wife Aoi and young daughter Hana, Sakamoto proves that even with his "dad bod," he has not lost his lethal edge. He improvises brutal and creative non-lethal takedowns using everyday items found in his store, from chopsticks that can catch bullets to refrigerators used as projectiles.

As the series progresses, Sakamoto finds himself building an unconventional family of reformed killers. He is joined by Lu Shaotang, the hot-headed and powerful daughter of a Chinese mafia boss, and Heisuke Mashimo, a gullible yet incredibly talented sniper with a pet bird. Together, this makeshift team defends their slice of ordinary life from the chaos of the underworld.

The narrative introduces several notable threats and arcs. The most immediate is the shadowy figure known as Slur, or Kei Uzuki, a rogue assassin who seeks to destroy the Japanese Assassins Association (JAA) and is responsible for the bounty on Sakamoto’s head. Sakamoto also faces adversaries from The Order, an elite group of assassins who maintain the underworld's balance. This includes his old JCC classmate Nagumo, a cheerful master of disguise, and other lethal fighters like the cynical Shishiba and the chainsaw-wielding Osaragi. As the story unfolds, deeper conspiracies emerge involving Sakamoto’s past at the JCC assassin training academy, forcing him to confront a world he thought he had left behind for good. The anime adaptation is noted for its stylized action sequences and a unique visual texture that mimics the look of paper, bringing the dynamic panels of the original manga to life.