In the Tokyo neighborhood of Shinjuku, a skilled assassin known as J roams the underworld as an untouchable force. Orphaned as a child and raised by the yakuza, he survived a betrayal that left three syndicates destroyed, earning him a reputation as a man not to be crossed. He cultivates a carefully curated hard-boiled persona, dressing in a black suit and trench coat, carrying a customized M1911A1 he calls J-Gov, and obsessively mimicking the tough-guy aesthetic of movies and novels. His real name is Yuichi Yamada. One day, a head-on collision with a young girl on the street causes their minds to swap bodies completely.

That girl is Megumi Akahoshi, a third-grade student at Shinjuku Ward Kinman Elementary School. She is half-Japanese and half-American, with silver hair and eyes. Megumi is academically gifted but socially isolated, bullied by classmates for her heritage and tormented at home by a mother who runs a strict, pressure-filled household. Her room is decorated with slogans like Anyone who can't keep time is garbage and Any score below 100 is goblin-level. Unable to endure the oppressive environment, she runs away from home, which leads directly to her fateful meeting with J.

After the body swap, Megumi discovers she prefers life as an adult assassin. She refuses to switch back, savoring the freedom of J's apartment and the ability to do as she pleases. J, now trapped in Megumi's small frame, faces a nightmare: he must attend elementary school while still carrying out assassination contracts. To maintain secrecy, he invents a cover story. He tells his fixer, Yanagi, that the girl in J's body is actually J's apprentice, a feral child named M. The fabricated backstory describes M as a foreign orphan who ate raw meat and killed from a young age, and who needs to murder periodically to stay sane. Yanagi buys the lie and starts supplying jobs to M, while J struggles to keep up with third-grade academics and a mother who terrifies even a hardened killer.

From this absurd foundation, a series of chaotic events unfolds. J, in Megumi's body, takes on assassination missions while riding a child's bicycle and using tricks from school life to stage hits. Meanwhile, Megumi, in J's body, finds that her natural charm and her love of a shoujo manga called Crazy Kiss give her an unexpected ability to disarm and seduce women, much to J's horror. Other characters complicate the situation: the female assassin Marie, who gets caught up in a temporary body swap with J; the middle-school gunsmith Mei, who maintains J's weapons; and Yanagi, whose fear of the supposed monster M grows. The more J tries to maintain his hard-boiled image, the more his real identity as a goofy, desperate man in a girl's body becomes apparent.

The story balances action sequences with comedy derived from the body-swap premise and J's earnest yet ridiculous pursuit of hard-boiled ideals. It also explores themes of family and belonging. J, who never knew a parent's love, is forced to navigate a mother-daughter relationship with Megumi's strict mother, Seiko, while Megumi, freed from that pressure, begins to take an interest in J's work and in the idea of justice. Notable arcs include the development of the M persona into an urban legend among the underworld, the temporary swap with Marie, and J's ongoing struggle to keep Megumi's academic record pristine while hiding his violent profession from her mother.

As of early 2026, the ongoing series has been collected into seven volumes, with an anime adaptation announced. The narrative continues to explore how an aging hit man and a lonely child adapt to each other's lives, each confronting the loneliness they once carried and finding unexpected connections in the most unlikely circumstances.