When Tenchi goes on a school trip to an archaeological dig, he and his friends accidentally stumble upon a mysterious underground cavern filled with ancient ruins. But as Tenchi, Amgasaki and Tsuchida begin to explore the ruins and search for an exit, they have no idea that the cavern is somehow connected to Yugi, the malevolent observer who continues to toy with the lives of Tenchi and the girls. And, wherever Yugi is involved, it seems certain that trouble for Tenchi cannot be far away…