Associate Professor Akira Takatsuki's Conjecture, Vol. 4 (manga)

“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!