"Sayonara, Oppai" is a manga series that explores themes of identity, self-acceptance, and societal expectations through a blend of humor and emotional depth. The story centers around Aoi Mutou, a high school student who struggles with the societal pressures surrounding body image, particularly due to her large bust size. Aoi's life takes an unexpected turn when she encounters a mysterious app that allows her to exchange physical attributes with others. Intrigued by the possibility of altering her appearance, Aoi uses the app to swap her breasts with those of her classmate, Riku, who has a flat chest and harbors insecurities about his own body.

As Aoi and Riku navigate their new physical realities, they begin to confront deeper issues related to self-worth, gender norms, and the expectations placed on them by society. The narrative delves into their personal struggles, highlighting the emotional toll of body dysmorphia and the desire for acceptance. Aoi, initially driven by a desire to escape the unwanted attention her body attracts, gradually learns to appreciate her own identity, while Riku gains confidence and a sense of empowerment through his new experiences.

The manga also introduces a supporting cast of characters, including classmates and family members, who provide additional perspectives on the central themes. These characters often serve as foils or mirrors to Aoi and Riku, further exploring the complexities of body image and societal judgment. The story balances lighthearted moments with poignant reflections, creating a narrative that is both entertaining and thought-provoking.

Throughout the series, the app's mysterious origins and consequences are gradually revealed, adding an element of intrigue to the plot. The ethical implications of altering one's body and the potential repercussions of such actions are explored, raising questions about the nature of identity and the pursuit of perfection. The manga concludes with a resolution that emphasizes self-acceptance and the importance of embracing one's true self, regardless of societal expectations.