Ordinary high-school
student Tenchi Masaki is spending his summer vacation at the family
shrine deep in the mountains of Okayama, training under the supervision
of his grandfather, the shrine keeper. On this particular day, Tenchi
has ignored his housekeeping duties and is searching the shrine office
for his grandfather's keys. It isn't long before he is caught red-handed,
and his grandfather challenges him to take the keys off him. Tenchi
lunges at his grandfather, but his attack is effortlessly blocked and
Tenchi himself sent crashing into a closet. As his grandfather leaves,
he reminds Tenchi that he must train harder and get on with his chores.
However, against all odds, Tenchi has managed to grab the keys!
Full of high spirits,
Tenchi heads for the forbidden cave near the shrine, a cave which has
been locked for many years. Legend tells of a demon that is sealed inside
the cave along with the sword that defeated it. Inside the cave, Tenchi
finds the legendary sword, its blade rusted and useless. Giving it an
experimental swing, Tenchi shatters the blade against a rock, which
splits down the middle, sealing the cave entrance and opening a secret
passage. With no other option, he ventures deeper into the cave, taking
the remains of the sword with him. 
He
soon comes to a huge underground chamber, and at the centre he finds
the mummified remains of the demon from the legend. To his horror, the
demon begins to move as he approaches. Too petrified to run, he stares
into the demons eyes which glow yellow behind the mask it wears. Suddenly,
the hilt of the sword, which he has tucked into his belt, glows brightly
as the demon touches him, and it emits a burst of light making the demon
stagger backwards. Spotting his opportunity, Tenchi flees the cave,
reassembles the sacred sealing rock by wrapping it in vines, and the
passageway closes behind him.
At school the next
day, Tenchi broods over his encounter with the demon. After a disagreement
with a classmate, he storms off to the school roof and dozes off. As
he sleeps, he dreams about his meeting with the demon the previous day.
When
he awakes at last, night has come, and he finds himself being watched
by a strange young woman. She introduces herself as Ryoko, and when
Tenchi spots her glowing yellow eyes, he realises she must be demon
from the cave. Ryoko tells him that, having been imprisoned in the cave
for 700 years, she is very angry, and wants to take out her frustrations
on him! Tenchi protests, telling her that it was his ancestor, Yosho,
that imprisoned her, but she attacks him nonetheless. The terrified
Tenchi makes a run for it, with Ryoko in hot pursuit hurling explosive
fireballs and a sword of light which she produces from a glowing red
gem on her wrist. Tenchi finally escapes her by hiding in one of the
school science labs, but finds that Ryoko's destructive rampage has
ruptured one of the gas pipes. He is about to make himself scarce when
Ryoko catches up with him. Cornered and helpless, Tenchi resorts to
the old "what's that outside the window" trick to distract
her and, when it actually works, makes a dash for it. When Ryoko realises
she has been tricked, the sparks from her weapon set off a gas explosion.
Tenchi assumes Ryoko must have been killed in the explosion, but his
relief is short-lived as she steps out of the flames, charred and blackened
and angrier than ever! However, luck is on his side; as Ryoko heads
for his current hiding place, she spots her reflection in a mirror and
stops to tidy herself up.
With time to think,
he realises that the gem on her wrist is the source of her terrifying
powers, and thinks he can easily beat her now. Alas, his loud exuberance
gives away his location, and Ryoko resumes her attack. 
He
flees via the window, but (literally) bumps into her again outside.
As she rushes to attack him, the remains of the legendary sword, which
he was keeping in his satchel, burst into life and protect him with
a blade of light. The sword cuts Ryoko's hand off, and the gem on her
wrist explodes. Tenchi apologises to her for cutting her hand off, but
with a grin Ryoko simply produces a new one from thin air. She then
vanishes, leaving Tenchi to do a disappearing act of his own before
he gets into trouble for blowing up the school!
Tenchi arrives back
home, exhausted after his combat with Ryoko. Thinking he will never
see her again, he slumps relieved onto his bed, only to find that Ryoko
is already waiting for him there, fast asleep under the covers!
