"First Love Monster" centers on Kaho Nikaidō, a 15-year-old high school student who moves to Tokyo to live independently after her parents relocate overseas. Upon arriving, she encounters Kanade Takahashi, a handsome and charming young man who saves her from an accident. Kaho quickly develops feelings for him, but her romantic aspirations take an unexpected turn when she discovers that Kanade is actually a fifth-grader. Despite his childlike appearance and age, Kanade exudes maturity and confidence, creating a comedic and awkward dynamic as Kaho grapples with her feelings.

The series explores the complexities of their unconventional relationship, blending humor with moments of genuine emotion. Kaho struggles to reconcile her attraction to Kanade with the reality of his age, while Kanade, despite his youthful demeanor, often displays a surprising level of emotional insight. The story delves into their interactions with a supporting cast of characters, including Kanade’s group of friends, who are also elementary school students but possess exaggerated personalities and quirks. These friends frequently add to the comedic tone of the series, often involving themselves in Kaho and Kanade’s relationship in unpredictable ways.

As the narrative progresses, Kaho’s determination to pursue her feelings for Kanade leads to a series of humorous and heartfelt situations. The series balances lighthearted comedy with occasional moments of introspection, touching on themes of love, societal expectations, and the challenges of unconventional relationships. While the premise is inherently absurd, the characters’ emotional sincerity and the series’ comedic timing keep the story engaging. The dynamic between Kaho and Kanade remains central, with their relationship evolving in unexpected ways as they navigate the complexities of their feelings and the reactions of those around them.