Kannazuki Yukariko lives alone in a super-mansion in the heart of Roppongi, Tokyo. Her age is unknown, but she is the older sister of a woman who would now be a grandmother. She believes that life is a grand form of killing time and that one should live beautifully. She loves graceful and elegant things and cannot tolerate anything ugly, boring, or that interferes with her enjoyment. The story begins when she spots a suspicious man trying to remove the baby carrier strap from a mother holding an infant in the street. Yukariko swiftly intervenes and scolds the man. When she returns to her opulent Western-style mansion, she finds a boy named Inori Mashiro waiting for her. Mashiro has lost both his parents and then his grandmother, who was Yukarikos younger sister, Midoriko. Yukariko agrees to let Mashiro stay, though she is unused to caring for others.

Soon, the household grows further. Chieri Hiyama, a genius elementary school girl who struggles with attending regular school, takes a liking to Yukariko and moves into the mansion. Kotaro Minamoto, a handsome con artist who initially tries to swindle Yukariko, instead has his abilities recognized and is hired as her butler. These three come to admire Yukariko and assist her in her missions.

Yukarikos philosophy is that life is a grand killing time, and she treats every day as an opportunity to eliminate whatever she finds ugly, boring, or disruptive. She confronts criminals, fraudsters, and other wrongdoers with a combination of immense wealth, sharp wit, and unconventional methods. Her interventions range from personally chasing down a thief on her vintage motorcycle to orchestrating elaborate stings using her own buildings and resources. She also has a softer side, such as when she helps stray cats or supports aspiring artists.

Recurring narrative arcs include Yukarikos tangled history with a man named Kabino, a charismatic villain who charms and ruins others, and her complicated feelings for a world-renowned violinist named Ninagawa, a longtime love interest who remains out of reach. The butler, Minamoto, has a past involving a butler café and old grudges that occasionally resurface. The story also explores Mashiros growth as he adjusts to life in Tokyo and Yukarikos eccentric world, Chieris development as she finds a place where her talents are valued, and the introduction of a university student named Minami who starts a young ladies club inspired by Yukariko.

Episodes are largely self-contained, often spanning two chapters, and feature a rotating cast of troubled people whom Yukariko helps in her signature lavish and stylish fashion. The setting shifts from the mansion to various Tokyo locations, including a convenience store that Yukariko briefly becomes owner of, a castle for a dress photography shoot, a zombie film set where she works as an extra, and a hot spring resort for a chaotic trip.