After locking herself in her closet for two years, Shijima Tsukishima finally emerges from isolation to attend high school. However, upon looking in the mirror, she discovers two shimeji mushrooms growing on top of her head. This strange occurrence, along with her lack of social skills and a history of being bullied in middle school, causes Shijima to keep to herself upon arriving at school. While reading alone, Shijima is approached by her classmate, Majime Yamashita, an eccentric girl with a fried egg on her head. Majime pesters Shijima to be her friend, giving her the nickname "Shimeji" after seeing the mushrooms on her head. After enough insistence on friendship, Shimeji gives in and decides to keep Majime company. Shimeji tries to navigate a less-than-normal high school life as she explores the mysteries and wonders of this strange world with her new tenacious companion.