Grief-stricken over the death of his last family member, his dog Hanako, picture book writer Hotaru Yuuyami suffers from writer's block for nearly two months. Despite his childhood friend and editor Tsubaki Uehara's best efforts, nothing seems to alleviate his pain. But all of this changes one day with the sudden apparition of Kinoko Inu, a mysterious mushroom from his garden that turns into a dog-shaped animal. Much to the great dismay of Tsubaki, Hotaru hastily invites the creature into his abode.

Days after Hotaru and Kinoko Inu start living together, the strange mushroom-dog decides to burn the last of Hanako's personal belongings in order to alleviate Hotaru's depression. However, the cohabitation between the overworked writer and his mischievous guest proves more difficult than expected, and Hotaru must let go of his grief if he wants to properly take care of Kinoko Inu.