Exploring dungeons in the real world is full of unexpected happenings! Suddenly, mysterious dungeons appear all over the world. High school student Taiki Kokonoe, together with his classmates Yukino and Yuuji, explores dungeons on a daily basis. Taiki and his friends struggle in battles with monsters and realize how tough it is to conquer dungeons. On the other hand, his younger sister Mika is very interested in dungeons...?