Tenchi Universe

Tenchi Masaki is a normal high school student in Japan, who spends the summers doing chores around his grandfather's family shrine. One day, Tenchi manages to steal the key to a mysterious cave where an oni (Japanese demon) is said to have been sealed away by his ancester Yoshou, whose sword still stands guard over it. When he enters this cave, he finds that, much to his surprise, the legend is indeed true when the oni's dessicated skeletal form ("mira"--Japanese mummy) comes to life and attacks him. Tenchi flees the cave in a panic, and later returns to high school, taking the hilt--seemingly all that remains of the ancestral sword--with him. At school, Tenchi is again attacked by the oni, whose name is Ryouko, who has now completely revived herself. Tenchi manages to defeat her with the sword's help, but this is only the beginning of a hilarious Japanese romantic comedy OAV (Original Animation Video, the Japanese equivalent of American made-for-video movies) series which has spawned several spin-off OAVs, soundtrack and drama CDs, and a new TV series in Japan.