"Watashi ni Tenshi ga Maiorita!" centers on the life of Miyako Hoshino, a reserved and socially awkward college student who struggles with self-esteem and forming connections with others. Her life takes an unexpected turn when her younger sister, Hinata, brings home her classmate, Hana Shirosaki, a cheerful and affectionate elementary school girl. Hana quickly becomes infatuated with Miyako, referring to her as an "angel" due to her kind and gentle demeanor. Despite Miyako's initial discomfort with Hana's overwhelming admiration, she gradually warms up to the child's presence, finding herself drawn into Hana's innocent and endearing world.

The series explores the dynamics between Miyako, Hana, and the people around them, including Hinata and Hana's older sister, Noa Shirosaki. Noa, a high school student, is initially skeptical of Miyako's interactions with Hana, fearing her intentions. However, as the story progresses, Noa begins to understand Miyako's genuine care for Hana and forms a bond with her as well. The narrative delves into themes of familial relationships, personal growth, and the impact of kindness, as Miyako learns to navigate her insecurities and open herself up to the connections she forms with Hana and others.

The interactions between the characters are lighthearted and often comedic, with Hana's boundless energy and adoration for Miyako serving as a catalyst for many humorous and heartwarming moments. The series balances these comedic elements with subtle emotional depth, particularly in Miyako's journey of self-acceptance and her evolving relationships with Hana, Hinata, and Noa. Through its focus on the bonds between the characters, "Watashi ni Tenshi ga Maiorita!" presents a wholesome and engaging story that highlights the transformative power of human connection.