Suzu Naito is a third-year high school student residing in the serene Kōchi Prefecture of Japan, grappling with a lost passion for singing and songwriting. The shadows of her past loom large over her—especially the memory of her mother, who tragically sacrificed her life to save a girl from a swollen river, leaving Suzu feeling abandoned. As a lingering connection to her mother's legacy, she maintains contact with a group of choir teachers who were her mother’s friends, but struggles to connect with her classmates. Only a select few, including her childhood crush, Shinobu Hisatake, the popular Ruka Watanabe, the athletic Shinjiro "Kamishin" Chikami, and her brilliant best friend, Hiroka "Hiro" Betsuyaku, offer her any semblance of companionship.
Encouraged by Hiro, Suzu dives into the vibrant virtual realm known as "U." Here, she creates a stunning avatar named "Bell," reflecting her own name in English through biometric analysis. The world of U rekindles her passion for music; with Hiro stepping in as her dedicated manager, Bell quickly ascends to fame, captivating audiences and earning the nickname "Belle," which translates to "beautiful" in French.
During one fateful concert, an infamous user known as "The Dragon" (or "The Beast") disrupts the event, pursued relentlessly by a vigilante group led by the zealous Justin. This group believes The Dragon threatens the tranquility of U and plans to expose his identity using an advanced program meant for a select few. As Suzu becomes intrigued by The Dragon’s mysterious persona, she learns about his surprising popularity among children who idolize him as a hero. As Suzu delves deeper, she is drawn closer to The Dragon and his five AI guardians.
In the real world, Ruka confides in Suzu about her crush on Kamishin, and with Suzu’s help, they navigate their feelings. However, in U, trouble brews as Justin captures and interrogates Belle, coercing her into revealing her identity under threat of exposure. Just when it seems like all hope is lost, The Dragon's AIs manage to rescue Belle, but not without consequence, as Justin’s group discovers The Dragon’s castle and sets it ablaze. Although Suzu longs to reach The Dragon, he vanishes, prompting her and Hiro to uncover his real identity before Justin can.
In their quest, they discover that The Dragon is actually a third-year junior high school student named Kei, who faces severe abuse from his father, yet fiercely protects his younger brother, Tomo. When Suzu sensitively reaches out to Kei, he struggles to believe her sincerity, haunted by a past where help has always fallen through. Over time, the support of her friends, including Shinjiro, Ruka, and the choir teachers, encourages Suzu to gain Kei’s trust through the power of her song.
In a heartfelt moment, Suzu reveals her identity to the world within U, performing with an emotional resonance that finally convinces Kei to reach out to her. Just when they seem on the verge of connection, Kei’s father discovers evidence of the abuse and cuts off their internet access, leaving Suzu desperate to find them. Using her detective instincts, Ruka and Kamishin help Suzu trace Kei's location back to Kawasaki, Kanagawa, near Tokyo. With authorities unable to act immediately, Suzu takes matters into her own hands, ensure Kei and Tomo's safety from their father's wrath.
In a powerful resolution, she reunites with her father at the station, and Shinobu commends her bravery, as Suzu embraces her newfound courage, ready to pursue the relationship she has longed for. Through this journey, Suzu ultimately finds peace with her mother's sacrifice, understanding the depth of her love, and paving a way towards healing in her own life.
