Fourteen-year-old Ichika Tachibana is a staid-natured junior high student who is given cleaning duty in an unused school building on the last day of school. Inside one of the rooms is a large mirror with a small crack rumored to harbor a ghost. Despite the rumors, Ichika goes to clean it—only to see some other figure in place of her own reflection! When she returns to the room to search for her cell phone, Ichika finds the figure outside of the mirror—a girl, with ponytails and a flowy dress, named Manatsu Kuroki.

Upon getting her cell phone back, one of the stones on Ichika's cell phone charm begins to shine, changing her outfit and allowing her to fly toward a being known as a Djinn. Ichika is then tasked to do Manatsu's homework assignment for her—to use the cell phone charm throughout the summer and write down her experiences with the Djinn she summons. Unbeknownst to her, Ichika's encounter with Manatsu will be the start of a summer she will never forget.