"Anthem of the Heart -Beautiful Word, Beautiful World-" centers on Jun Naruse, a high school girl who has been mute since childhood due to a traumatic incident. As a young girl, Jun inadvertently caused her parents' divorce by revealing her father's affair, an event that led her to believe her words bring harm to others. Consequently, she is cursed by a magical "egg fairy" and loses her ability to speak. Years later, Jun is tasked with participating in a community outreach project alongside her classmates, which involves creating a musical performance. Among her peers are Takumi Sakagami, a reserved and thoughtful boy; Natsuki Nitō, a cheerful and outgoing girl; and Daiki Tōma, a former baseball player struggling with his own insecurities.

The narrative explores Jun's journey as she navigates her inability to speak and her growing connection with her classmates. Through the musical project, Jun discovers that she can express herself by singing, a revelation that becomes pivotal to her emotional healing. The group's collaboration on the musical fosters deeper bonds, as each character confronts their personal struggles. Takumi, who harbors feelings for Natsuki, grapples with unrequited love, while Natsuki wrestles with her own feelings of inadequacy. Daiki, on the other hand, faces the pressure of living up to expectations after an injury ends his baseball career.

As the story progresses, Jun's interactions with her peers help her gradually overcome her fear of speaking. The musical becomes a metaphor for communication and self-expression, highlighting the importance of confronting one's emotions and vulnerabilities. The climax of the story occurs during the performance, where Jun's singing serves as a cathartic moment, symbolizing her reclaiming of her voice and her ability to connect with others. The resolution sees the characters reflecting on their growth, with Jun taking tentative steps toward a future where she no longer feels bound by her past trauma. The series emphasizes themes of healing, communication, and the transformative power of art.