G.A.P ~Unknown Post Office~

High school student Naoto Maji (Naoto) receives a letter from Miku Himejima, who has been appointed as a part-time lecturer, with the word "lost child" written on the front. A mysterious old man Rogetsu appears in front of Naoto who finds out the location of the " Ichikokubashi " that was inside . Rogetsu gives him a letter and tells him to solve the mystery. It was the first letter of the spirit mail inviting Naoto to become involved with Koyori, the cursed god, and the "Spirit Legger".