"Hatsukoi Monster" centers on Kaho Nikaidō, a 15-year-old girl from a wealthy family who moves into a new apartment complex after her father’s financial downfall. While adjusting to her new life, she encounters a young boy named Kanade Takahashi, who saves her from a falling construction beam. Grateful and struck by his kindness, Kaho develops feelings for him, only to discover that Kanade is a fifth-grader. Despite the significant age gap, Kaho struggles to reconcile her emotions, leading to a series of comedic and awkward situations as she navigates her first love.

Kanade, though physically a child, possesses a mature and confident demeanor, often surprising those around him with his straightforward and sometimes flirtatious behavior. His interactions with Kaho are marked by a mix of innocence and unintentional charm, which further complicates her feelings. The series explores the dynamics of their unconventional relationship, as well as the reactions of those around them, including Kaho’s friends and Kanade’s classmates.

The narrative delves into themes of first love, societal expectations, and the complexities of human emotions. Kaho’s internal conflict is a central focus, as she grapples with her feelings for Kanade while facing judgment and confusion from others. The series balances lighthearted humor with moments of introspection, creating a tone that is both playful and thought-provoking. Supporting characters, such as Kaho’s childhood friend Atsushi Taga and Kanade’s protective older brother, add depth to the story, providing additional perspectives on the central relationship.

"Hatsukoi Monster" presents a unique premise that challenges conventional romantic tropes, blending comedy and romance with a touch of absurdity. The story unfolds through a series of episodic encounters, gradually revealing the personalities and motivations of the characters. While the premise may be polarizing, the series maintains a focus on the emotional journey of its protagonist, offering a distinctive take on the complexities of love and attraction.