"Oppai Heart: Kanojo wa Kedamono Hatsujōki!?" is an ecchi anime series that centers around the life of Haruto Kamijou, a high school student who becomes entangled in a series of comedic and risqué situations after encountering a mysterious girl named Aina Hiragi. Aina is revealed to be a "beast hormone girl," a unique individual whose body produces a powerful pheromone that induces uncontrollable lust in those around her. This pheromone effect creates a chaotic environment, particularly in Haruto's school, where students and teachers alike are affected by Aina's presence.

Haruto, despite being initially overwhelmed by the bizarre circumstances, finds himself drawn to Aina and takes on the responsibility of protecting her from the unintended consequences of her condition. As the story progresses, other characters are introduced, including additional "beast hormone girls," each with their own distinct personalities and quirks. These characters contribute to the escalating comedic and romantic tension, as Haruto navigates his relationships with them while trying to maintain some semblance of normalcy in his life.

The narrative explores themes of attraction, desire, and the challenges of managing relationships in the face of extraordinary circumstances. The series balances its ecchi elements with lighthearted humor and moments of character development, as Haruto and the girls confront their feelings and the societal implications of Aina's condition. The plot unfolds through a series of episodic encounters, often driven by misunderstandings and the unpredictable effects of the pheromones, leading to a mix of comedic and dramatic situations.

The main characters include Haruto Kamijou, the protagonist who serves as the grounding force amidst the chaos; Aina Hiragi, the central "beast hormone girl" whose presence drives much of the plot; and a supporting cast of other hormone girls and classmates who add depth and variety to the story. The interactions between these characters form the core of the series, as they navigate the challenges posed by Aina's condition and their own evolving relationships.