Manga: Associate Professor Akira Takatsuki's Conjecture, Vol. 4

If you had died, professor I would be all alone again. The truth behind Mana, the Miracle Girl of Okutama, comes to light! And soon after, the crew ventures out to investigate a mysterious cave said to enshrine an oni. There, they find not relics of a demon, but rather a human skull with a gaping hole... All this and more, as Professor Takatsuki and Naoya risk life and limb in their hunt for the supernatural!