"Nichiyōbi ni Kanojo wa" is a manga series that explores themes of love, identity, and societal expectations through its central narrative. The story revolves around a young man named Takuya, who leads a double life. By day, he is an ordinary office worker, but on Sundays, he transforms into a woman named Aya. This transformation is not merely a disguise; Takuya fully embraces his female identity, adopting Aya's mannerisms, appearance, and lifestyle. The duality of his existence becomes the focal point of the series, as it delves into the complexities of his relationships and the challenges he faces in reconciling his two identities.

The plot intensifies when Takuya, as Aya, meets and falls in love with a man named Shōta. Shōta is unaware of Aya's true identity, believing her to be a woman. This creates a poignant tension as Takuya navigates the emotional and ethical dilemmas of maintaining the relationship under false pretenses. The series examines the consequences of Takuya's choices, particularly as his feelings for Shōta deepen and the risk of exposure grows. The narrative also explores Shōta's perspective, highlighting his genuine affection for Aya and the confusion he experiences when confronted with the truth.

Supporting characters add depth to the story, including Takuya's childhood friend, Yūki, who is aware of his dual identity and provides emotional support. Yūki's role is pivotal in grounding Takuya and offering a lens through which the audience can understand the societal pressures and personal struggles Takuya endures. The series also touches on broader themes of gender identity and acceptance, portraying the challenges faced by individuals who do not conform to traditional gender norms.

The manga balances moments of lightheartedness with emotional weight, creating a narrative that is both engaging and thought-provoking. It avoids simplistic resolutions, instead presenting the complexities of human relationships and the nuances of identity. Through its exploration of love, deception, and self-discovery, "Nichiyōbi ni Kanojo wa" offers a compelling and multifaceted story that resonates with readers.