"Kokuriko-Zaka Kara" is a manga series set in 1963 Yokohama, Japan, during a period of rapid modernization and cultural change. The story centers on Umi Matsuzaki, a high school girl who lives in a boarding house called Coquelicot Manor, located on a hill overlooking the harbor. Umi manages the household in her mother's absence, balancing her responsibilities with her studies and friendships. Her daily routine includes raising signal flags each morning, a tradition she maintains in honor of her father, a sailor who died during the Korean War.

Umi's life takes a turn when she meets Shun Kazama, a fellow student and member of the school's newspaper club. Shun is investigating the history of Coquelicot Manor, which is at risk of being demolished to make way for new development. As Umi and Shun work together to uncover the building's past, they discover connections between their families that challenge their understanding of their identities. The narrative explores themes of heritage, memory, and the tension between preserving tradition and embracing progress.

The supporting cast includes Umi's younger siblings, Sora and Riku, who help manage the boarding house, and various residents of Coquelicot Manor, each contributing to the story's exploration of community and resilience. The manga also delves into the lives of Umi and Shun's classmates, highlighting the aspirations and struggles of a generation navigating a rapidly changing society.

The plot intertwines personal and historical elements, with the impending demolition of Coquelicot Manor serving as a catalyst for Umi and Shun's journey. Their efforts to save the building reveal layers of its significance, not only as a physical structure but also as a symbol of continuity and shared history. The story concludes with a resolution that balances the inevitability of change with the importance of honoring the past, leaving a lasting impression of hope and connection.