"Love Is the Number of Keys" centers on Aoi Nishimori, a reserved and introverted high school student who works part-time as a pianist at a local hotel. Aoi’s life is marked by a deep connection to music, which serves as both an escape and a means of self-expression. The narrative unfolds as Aoi encounters various individuals who influence her journey, each tied to a specific piano key that symbolizes their emotional impact on her life.

The story begins with Aoi’s routine performances at the hotel, where she meets a diverse group of guests and staff. Among them is Shusei Kudo, a talented but enigmatic composer who recognizes Aoi’s potential and challenges her to explore new musical horizons. Their interactions gradually reveal Shusei’s own struggles with creativity and personal loss, creating a bond rooted in mutual understanding. Another key figure is Ritsu Mizushima, a cheerful and outgoing classmate who encourages Aoi to step out of her comfort zone and embrace life beyond the piano. Ritsu’s optimism contrasts with Aoi’s reserved nature, leading to moments of growth and self-discovery.

As the series progresses, Aoi’s relationships deepen, and the narrative explores themes of love, identity, and the transformative power of music. Each character’s story intertwines with Aoi’s, highlighting how their presence resonates with her emotionally and artistically. The concept of “keys” serves as a metaphor for the connections Aoi forms, with each key representing a pivotal moment or relationship that shapes her journey.

The plot balances Aoi’s personal growth with her evolving relationships, culminating in a climactic performance that symbolizes her emotional and artistic maturity. Through its exploration of music and human connections, "Love Is the Number of Keys" presents a heartfelt narrative that emphasizes the importance of understanding oneself and others. The series maintains a focus on character development and emotional depth, offering a nuanced portrayal of its central themes.