Oriko Fujidou, the young heiress to the Fujidou Zaibatsu, was given an entire city as a gift by her father on her fourth birthday. For the next ten years, she devoted herself to its growth and prosperity. Her peaceful days as the city's guardian come to an abrupt end when a giant unidentified invader, a kaiju, attacks without warning. In a desperate attempt to fight back, her loyal and eccentric butler, Dr. Sebastian, gives Oriko an experimental energy drink from his research and development team. The moment she drinks it, her body grows to a colossal size, towering over skyscrapers. After a clumsy struggle, she manages to repel the invader, though not without accidentally causing significant destruction to the city herself. She then declares her intention to always defend the town as its giant protector. However, the attacks do not stop. Oriko is forced to transform again and again as new threats emerge, turning her into a recurring giant savior for the city she cherishes, even as her enormous size continues to wreak havoc on its infrastructure.

The story follows Oriko, a capable and wealthy young woman who is nonetheless prone to embarrassment over the revealing situations her transformations often cause. She is aided by Dr. Sebastian, whose species has a symbiotic drive to serve a master, and her security chief, Madoka. Their quiet lives are disrupted by the arrival of a persistent rival, Stella, a childish but powerful alien invader whose plans to conquer the city frequently backfire, reducing her to a sitcom-style nemesis who often ends up as an unlikely ally. Other notable arcs involve the introduction of Midori, a bodyguard with secret fantasies of giant destruction; the arrival of a young prodigy named Nakano Giraffe who idolizes Oriko; and the eventual emergence of a more sinister force behind the invaders, potentially tied to Oriko's own mysterious alien heritage. The series maintains a comedic and wholesome tone, avoiding graphic violence and focusing on the absurdity and chaos that ensue whenever a wealthy young lady grows to the size of a skyscraper to solve her problems.