A former teacher named Takeishi, fired from his previous job, stumbles onto a mysterious website and ends up being recruited as a teacher at Hittoman Girls' High School, a top-secret assassin training facility located on a remote island. Due to his intimidating appearance and a series of pure coincidences, Takeishi is immediately (and incorrectly) identified as the legendary assassin whom every killer in the underworld admires. In reality, he is a completely ordinary man with no combat skills, no special training, and no desire to kill anyone. His true identity must never be exposed, or he will be killed instantly.

The school is a lethal environment where even walking down a hallway can be deadly, and the students and faculty are all highly trained killers. Takeishi's mundane actions—tripping, sneezing, shouting in surprise—are consistently misinterpreted as masterful feints, tactical dodges, or intimidating auras. This core misunderstanding fuels the comedy as he stumbles from one life-threatening situation to another, somehow surviving through luck and the preconceived notions of those around him.

The key supporting character is Nagaiizumi, a perceptive student who discovers Takeishi's secret early in the story. Rather than exposing him, she becomes his reluctant confidante and, later, his sole ally in maintaining the ruse. Her knowledge of his true nature adds a new layer of tension and strategic maneuvering to the narrative.

The story unfolds in three major arcs. The first arc establishes Takeishi's arrival and his initial survival through a series of escalating misunderstandings. The second arc sees him trapped in a forest rigged with lethal booby traps, which he miraculously survives by accident, only to have Nagaiizumi confront him with the truth. The third and final arc culminates in a chaotic battle royale where fifteen students, all of them assassins, compete to become Takeishi's bride—a marriage contest initiated by Nagaiizumi as a desperate plan to protect him. The manga concludes with Takeishi's fate left hanging as the competition unfolds.