"Natsu Koi" is a romantic comedy manga set during the summer, centering on the relationship between Natsuki Iijima, a high school student with a reputation for being aloof and unapproachable, and Ryouhei Arima, a cheerful and outgoing classmate. The story begins when Natsuki, who is often misunderstood due to her sharp tongue and reserved demeanor, is tasked with organizing the school's summer festival. Ryouhei, known for his sociable nature and popularity, volunteers to help her, sparking an unexpected connection between the two.

As they work together, Natsuki's guarded personality gradually softens, revealing her thoughtful and caring side. Ryouhei, intrigued by her complexity, becomes determined to understand her better. Their interactions are marked by playful banter, misunderstandings, and moments of genuine emotional vulnerability. The narrative explores themes of self-discovery, communication, and the transformative power of relationships, as both characters learn to navigate their feelings and insecurities.

Supporting characters, such as Natsuki's close friend Yuka and Ryouhei's childhood friend Hiroshi, add depth to the story by providing contrasting perspectives on love and friendship. The summer setting, with its festivals, fireworks, and beach outings, serves as a backdrop for the evolving romance, emphasizing the fleeting yet impactful nature of the season.

The plot balances lighthearted humor with poignant moments, capturing the awkwardness and excitement of young love. Natsuki and Ryouhei's relationship progresses through a series of small, meaningful events, from shared lunches to late-night conversations, culminating in a heartfelt confession. The manga concludes with a sense of closure, leaving the characters' futures open to interpretation while celebrating the growth they experienced together.