Emerging from a two-year seclusion in her closet, Shijima Tsukishima ventures into high school only to find two shimeji mushrooms sprouting atop her head in the mirror. Haunted by past bullying and a quiet nature, Shijima's world shifts as the peculiar classmate Majime Yamashita, donning a fried egg crown, insists on befriending her, dubbing her 'Shimeji.' Reluctantly, Shimeji gives in to the persistent friendship overtures and embarks on an offbeat high school journey, navigating the peculiarities of life alongside her quirky and unwavering companion.