"Say 'I Love You'" centers on Mei Tachibana, a socially withdrawn high school student who has distanced herself from others due to past betrayals and bullying. Preferring solitude, Mei avoids forming connections until an unexpected encounter with Yamato Kurosawa, a popular and outgoing classmate, disrupts her isolated routine. After a misunderstanding leads to Mei kicking Yamato, he becomes intrigued by her aloofness and begins pursuing a friendship, eventually expressing romantic interest. Despite her initial reluctance, Mei gradually opens up to Yamato, marking the beginning of her journey toward self-discovery and emotional vulnerability.

The narrative explores the complexities of their relationship as Mei navigates her insecurities and learns to trust others. Yamato’s unwavering support helps her confront her fears, but their bond is tested by external pressures, including misunderstandings, jealousy, and the interference of other characters. Among these are Yamato’s childhood friend, Aiko Muto, who harbors unrequited feelings for him, and Mei’s classmate, Asami Oikawa, who struggles with her own romantic dilemmas. The series delves into themes of friendship, self-worth, and the challenges of communication, portraying the emotional growth of its characters as they grapple with love and personal identity.

Mei’s transformation from a guarded individual to someone willing to embrace vulnerability forms the core of the story. Her relationship with Yamato serves as a catalyst for her development, but the series also highlights the importance of other relationships in her life, such as her friendship with Asami and her evolving dynamic with classmates. The plot balances moments of lighthearted romance with deeper explorations of emotional struggles, creating a narrative that resonates with themes of healing and connection. Through its portrayal of teenage relationships and personal growth, "Say 'I Love You'" offers a heartfelt exploration of love and self-acceptance.