"U" is a popular social media platform where people can create a virtual persona and start a new life. Among its five billion users, one newcomer is quickly gaining attention: Belle, a beautiful singer whose alluring melodies slowly capture the hearts of the masses. But in this space where everyone hides behind an avatar, curiosity arises over who the mysterious girl truly is.

Suzu Naito—a shy girl from the countryside—can no longer sing following past trauma, all her efforts resulting in breakdowns and illness. However, when Suzu joins U, she is once again able to project her voice. Under the alias "Belle," her vocals soon go viral, receiving both love and hatred. Meanwhile, rumors spread of a chaotic beast within U, known only as "The Dragon." After a chance meeting during her concert, Belle finds he is not as evil as the stories suggest. Now, both online and in the real world, Suzu has to face the struggles of identity, fame, and opening one's heart.