"Hitotsuboshi-ke no Ultra Baa-san" centers on the daily life of the Hitotsuboshi family, particularly focusing on the grandmother, Ultra Baa-san, whose eccentric and larger-than-life personality drives much of the series' humor and charm. Ultra Baa-san is a 91-year-old woman with an unyielding spirit, boundless energy, and a penchant for mischief. Despite her age, she possesses superhuman strength and a mischievous streak that often leads to chaotic yet heartwarming situations. Her antics are both a source of joy and exasperation for her family, particularly her grandson, Hiroshi, who often finds himself caught up in her unpredictable schemes.

The series is structured as a slice-of-life comedy, with each episode presenting self-contained stories that highlight Ultra Baa-san's unique approach to life. Whether she is challenging younger generations to physical feats, outsmarting her neighbors, or embarking on absurd adventures, her actions consistently defy expectations of what a nonagenarian should be capable of. Her interactions with Hiroshi and other family members, including her daughter-in-law and great-grandson, provide a humorous yet affectionate portrayal of intergenerational dynamics.

Ultra Baa-san's character is defined by her unwavering optimism, resourcefulness, and a deep love for her family, even if her methods of expressing it are unconventional. The series balances its comedic elements with moments of genuine warmth, showcasing the bonds within the Hitotsuboshi family. Through its episodic format, "Hitotsuboshi-ke no Ultra Baa-san" explores themes of aging, family, and the joy of living life to the fullest, all while maintaining a lighthearted and entertaining tone. The narrative thrives on the contrast between Ultra Baa-san's extraordinary abilities and the mundane, everyday settings in which her adventures unfold, creating a unique and engaging viewing experience.