Hayato Marikoji, a 24-year-old debt-ridden man, accepts a teaching position at Shinobi Gakuen ninja academy in Kasumi Kogen to repay a two-million-yen obligation inherited from a friend. Unaware the school exclusively trains female kunoichi and that the village operates on ninja principles, he attempts resignation upon discovery. This plan halts when he saves student Himawari Hinata from an oncoming train.

Lacking formal ninja abilities, Hayato endures constant peril through sheer luck, an uncanny knack for dodging attacks, and protective students who shield him. His sole combat skill—proficient dart throwing—occasionally aids with kunai handling. Crippling financial strain defines his existence; village-mandated salary deductions leave him nearly destitute, driving relentless hunts for food and side income.

A shuriken-shaped mark on his neck convinces Himawari he is her childhood savior, forging her unwavering loyalty and self-assigned role as his guardian. He becomes entangled in village schemes, including repeated assassination attempts by a faction led by "The Boss," who erroneously believes he descends from her ancestors' killer. These attempts consistently fail due to absurd mishaps or Hayato's accidental presence, eventually resolving her vendetta. The second season delves deeper into his undisclosed past.

Initially met with skepticism, Hayato gradually earns mutual respect and camaraderie with the kunoichi. He remains an ordinary man navigating a clandestine world of supernatural threats and financial desperation.