Chihiro, who loves mythology, goes to the laboratory of Professor Tsukishima, whom he admires. There, he meets a mysterious professor and three beautiful women. At Tsukishima's urging, Chihiro draws a picture of a certain creature. This is the true form of the monster "INner" (the thing inside) that lurks inside people. She is told by Tsukishima that his memory made him draw it... The monster appears in front of Chihiro, who can't hide her confusion!?

"After School! Dungeon High School" is being serialized in the monthly COMIC Ryu! Masanori Yamanishi, the long-awaited first book of the phantom early works is finally gathered at the suspicious antique shop "Kuromori" which is just a recycle shop where Mako Kuromori, who doubles as a shopkeeper, Yuko Monobe, who is still wearing a Noh mask, and Satsuki Seike, a high school girl who aims to change her mind from a shackle, live together with a strange magic item!