The narrative of Mushi-uta is set in the near future. A decade prior to the story's beginning, peculiar insectoid entities referred to as "Mushi" started to emerge. These Mushi can devour people's dreams and thoughts, granting supernatural abilities in exchange. The protagonist, Daisuke "Kakkou" Kusuriya, meets a young girl called Shiika Anmoto. Gradually, the pair grow quite intimate. Yet, without Kakkou's knowledge, Shiika is a fugitive from a clandestine detention center named GARDEN, which houses individuals possessed by Mushi, termed Mushitsuki. GARDEN's armed wing, the Special Environmental Conservation Executive Office, sends its top assassin to hunt Shiika down. Their mission, however, meets opposition from the Mushibane resistance group, commanded by the enigmatic "Ladybird".