Set in the virtual online world "U" with more than 5 billion users around the world, the protagonist "Bing" is a 17-year-old high school girl living in the countryside who lost her mother when she was young and left psychological trauma. She, who once liked to sing with her mother, no longer opened her mouth to sing, and became estranged in her daily relationship with her father. One day she came into contact with another world, the virtual world "U", and participated in the name "BELL". When BELL sang again in "U" and released her own songs, she gained great popularity. At this time, the existence of the frightening mystery in "U" appeared in front of her in the form of a dragon.