A former hikikomori who lives in a danchi with her older sister, Shijima decides to leave her closet after two years and start her term as a high school student. However, due to hiding away from society for a long time, she is seen with two shimeji mushrooms sprouting on top of her head, earning the nickname "Shimeji-chan" from a certain girl with a fried egg. Shijima is aloof, sarcastic and initially unwilling to make friends in sharp contrast to Majime, but she eventually warms up to her classmate's tiresome antics and the two become friends.