Hidomi Hibajiri is a dissilusioned young girl who never takes off her headphones. Her whole life consists of going to school, helping out at her mother's cafe, and listening to music. And with nothing else to break the crippling monotony, she keeps her headphones on at all times. That is, until she is run over by a mysterious guitar-wielding woman.

Everyone in the world uses smartphone apps. But inside them lurks unknown creatures called "Appli Monsters," or "Appmon." The Appmon are AI lifeforms with the ability to think and act, and exist in the boundary between the human world and digital space. In the vast sea of the internet, the "last boss AI" Leviathan takes control of the Appmon with a