Everyone around the world uses smartphone applications, yet undiscovered beings known as "Appli Monsters" or "Appmon" hide within them. These creatures possess artificial intelligence allowing them to think and act, existing in the in-between of the human world and digital landscape. Within the vastness of the internet, the "last boss AI" Leviathan infects Appmon with a virus, gaining control over every system and initiating power over the human world through the digital realm. Haru Shinkai becomes the possessor of the Appli Drive and materializes Gatchmon, a monster search application, to combat the virus.