A hitman's muscle memory trapped in a kid's body, forced to blend in at middle school while his brain is still running assassination protocols. The comedy comes from watching a 39-year-old war machine navigate cafeteria politics and homework with the same lethal precision he once used on crime syndicates. Every awkward classroom moment is a man who has killed for a living realizing that surviving thirteen-year-olds might actually be his hardest mission yet.

Juuzou Oogami is a famed assassin with a perfect record, though he holds to a personal rule of killing only people who are truly evil. Right after he destroys a powerful organization, a strange wasp stings him and he passes out. When he comes to, his 39-year-old body has shrunk into that of a 13-year-old boy. Before he can make sense of it, his boss gives him a fresh assignment suited to his new size: go undercover at a private middle school, where the boss plans to enroll his daughter the following year. Having started killing young, Juuzou never had a normal childhood, and he soon finds himself enjoying schoolwork despite a rough first impression on his classmates. He also learns that his classmate Noren Mitsuoka is linked to the company behind the wasp. As assassins close in, Juuzou makes friends, savors ordinary teenage life and hunts for a way to get his old body back. The result is an action comedy built on this odd second youth.