"That Blue Summer" is a coming-of-age manga that explores themes of friendship, self-discovery, and the complexities of adolescence. Set in a small coastal town, the narrative centers on a group of high school students during a transformative summer. The protagonist, a reserved and introspective teenager, forms an unlikely bond with a charismatic and free-spirited transfer student. Their friendship becomes the catalyst for a series of events that challenge their perceptions of themselves and the world around them.

The story unfolds through a series of vignettes that capture pivotal moments in the characters' lives. The protagonist struggles with feelings of inadequacy and a desire to break free from societal expectations, while the transfer student harbors a mysterious past that gradually comes to light. Their interactions are marked by moments of vulnerability, humor, and conflict, as they navigate the uncertainties of youth.

Secondary characters, including classmates and family members, play significant roles in shaping the narrative. A childhood friend of the protagonist serves as a grounding presence, offering support and perspective, while a rival student introduces tension and competition. The dynamics between these characters highlight the complexities of relationships during adolescence, as they grapple with identity, ambition, and the passage of time.

The coastal setting serves as a metaphor for the characters' emotional journeys, with the vastness of the ocean reflecting their inner turmoil and aspirations. The summer season, with its fleeting nature, underscores the transient yet impactful nature of their experiences. As the story progresses, the characters confront their fears, confrontations, and moments of clarity, leading to personal growth and a deeper understanding of themselves and each other.

The manga balances lighthearted moments with poignant reflections, creating a narrative that resonates with the universal challenges of growing up. Through its exploration of friendship, self-acceptance, and the search for meaning, "That Blue Summer" offers a nuanced portrayal of adolescence, capturing the bittersweet beauty of a pivotal season in life.