"Himouto! Umaru-chan" centers on the daily life of Umaru Doma, a seemingly perfect high school girl admired by her peers for her beauty, intelligence, and athleticism. However, at home, Umaru transforms into a lazy, self-indulgent otaku who spends her time playing video games, binge-watching anime, and consuming junk food. This stark contrast between her public and private personas drives much of the series' humor and character dynamics.

Umaru lives with her older brother, Taihei Doma, who works a demanding office job and often finds himself cleaning up after her or catering to her whims. Despite his frustrations, Taihei cares deeply for his sister and supports her, even as her antics disrupt his life. The siblings' relationship is a core element of the story, balancing moments of comedy with genuine warmth and familial bonds.

The narrative expands to include Umaru's friends and classmates, such as the kind and responsible Kirie Motoba, the cheerful and energetic Sylphinford Tachibana, and the reserved but observant Ebina Nana. These characters add depth to the story, as their interactions with Umaru reveal different facets of her personality and create opportunities for growth. Kirie, in particular, often finds herself drawn into Umaru's schemes, while Sylphinford and Ebina provide contrasting perspectives on friendship and individuality.

The series explores themes of identity, self-acceptance, and the balance between personal desires and social expectations. Umaru's dual nature serves as a metaphor for the pressures of conforming to societal standards while secretly indulging in personal passions. Over time, Umaru begins to reconcile these aspects of herself, gradually becoming more considerate of others and learning to navigate her relationships with greater maturity.

The humor in "Himouto! Umaru-chan" is largely situational, deriving from Umaru's exaggerated laziness, her brother's exasperation, and the absurdity of her otaku lifestyle. The series also incorporates moments of heartfelt sincerity, particularly in its portrayal of sibling bonds and the challenges of growing up. Through its blend of comedy and character development, "Himouto! Umaru-chan" offers an entertaining and relatable exploration of everyday life, friendship, and the quirks that make individuals unique.