Umi, a 16-year-old student in 1960s Yokohama, lives at Coquelicot Manor, a boarding house shared with her grandmother, younger siblings, and tenants. Each dawn, she raises maritime signal flags in a ritual inherited from her father, a naval captain who perished when his ship hit a mine during the Korean War. The flags, alongside fresh flowers and water offered at her father’s shrine, honor his memory and channel her grief into a quiet plea for safe journeys.

Tasked with overseeing her siblings and household duties in her mother’s absence—pursuing medical studies abroad—Umi embodies steadfast responsibility, juggling schoolwork, chores, and emotional restraint. Her structured life shifts when Shun Kazama, a bold classmate campaigning to salvage their school’s crumbling Latin Quarter clubhouse, draws her into his cause. Though initially skeptical of his reckless enthusiasm, she aids his efforts, devising renovation plans and inspiring peers to rally behind the project, earning the moniker “goddess of good luck.”

As their collaboration deepens into mutual affection, a discovered photograph hints at a possible sibling connection—Shun’s adoption ties him to her father’s history. The revelation fractures their bond, prompting Shun’s withdrawal and Umi’s struggle to reconcile her emotions with perceived familial duty. Their tentative reconciliation culminates in a shared admission of feelings, later validated by Yoshio Onodera, a surviving comrade of her father, who confirms Shun’s biological parents were unrelated to her family.

Emerging from these trials, Umi transitions from a guarded caretaker to someone embracing vulnerability. The clarified truth about her father’s legacy and Shun’s lineage allows her flag-raising ritual to transform into a symbol of forward-looking hope. Her compassionate leadership, evident in uniting classmates to preserve the Latin Quarter and nurturing connections with family and tenants, underscores her growth into a figure who bridges duty with open-hearted resilience.