Tenchi Muyo is basically the story of another anime high school kid, Tenchi Masaki, who ends up with a bunch of aliens living with him. If you are an anime fan who has actually managed to not know anything about the series, I'd recommend doing yourself a favor and just skiping the rest of this summary.
Tenchi Masaki appears to be a relatively normal high school kid (if you don't count the fact that his grandfather is the caretaker for an isolated shrine and is training Tenchi in some sort of ancient martial art), and his life is pretty quiet. Quiet, that is, until he decides to disobey his grandfather's orders and ventures into a cave near the shrine. The cave is rumored to hold a demon that terrorized the countryside centuries ago, as well as the sword used to subdue it--and the cave doesn't disappoint. Well, the demon isn't quite what it appears to be, and before long Tenchi is caught in the middle of a feud between Ryoko, a superpowerful space pirate and Ayeka, princess of the planet Jurai. As the series progresses, more (attractive, female) misfits happen upon poor Tenchi, and he ends up with quite a harem of live-in aliens. The supporting cast includes: Mihoshi, a ditzy but diligent Galaxy Police officer, Sasami, Ayeka's younger sister, Washu, a diminutive mad scientist and the greatest genius in the galaxy, Ryo-ohki, the cute and cuddly offspring of Ryoko¹s spaceship (no sarcasm at all), Tenchi's enigmatic grandfather, and Tenchi's dad, who's only hobby seems to be peeping at the many new female acquaintances of his son. This whole crew has only two things in common: they all seem to have the hots for Tenchi for some reason (father and grandpa excepted, of course), and they're all living in Tenchi's house.
In the first OAV series, Earth Chapter, the plot begins with the main characters being introduced to each other through a series of humourous misadventures. Later, we are introduced to Kagato, a rather unpleasant intergalactic criminal who (several milinia ago) was a student of Washu. He's come to discover the powerful secrets of the Jurai Royal Family (via Tenchi and friends) so he can take over the universe or some such villany. Much violence, of course, is sure to result.
Then there's a little interlude, the Mihoshi Special, that's totally unrelated to the rest of the plot--it involves a superhero, Pretty Sammy, who bears a striking resemblance to a certain popular transforming sailor. This superheroine is none other than Sasami's heroic alter ego. Well, at least it's Sasami's alter ego in Mihoshi's little tale of one of her great Galaxy Police exploits--she tells the story using her friends (Tenchi and crew) as stand-ins for the real heroes and villians. This story was spun off into an entire Pretty Sammy series... but that's another story.
Well, after that little sidestep, we launch into the second series, Space Chapter. We start with a couple of episodes that delve into the past of some of the crew, and then we're introduced to a whole new villian. This fellow is a lot less menacing than Kagato, but he's also an old nemesis of Washu, and this guy's got powerful (and quite mysterous) backing, in the form of the entity Tokimi. Cruzing around in a ship called the Goddess, he cooks up a clone of Ryoko and sends her off to replace the real one and take care of Tenchi. After that, we end the series with a light story wherein we get to meet some more of the Jurai royal family.<br />