Our heroine is Miaka (Yuuki Miaka), a (somewhat) gluttonous but kind-hearted and cheerful junior-high student in danger of not passing her high school entrance exams. She is, naturally, suffering through the hellish "jyuken" study regimen that so many young Japanese children suffer. Her best friend is Yui, a fellow student. The two girls enter the magical world and through a series of adventures wind up on different sides of a conflict. Through bad luck, lies, and her own jealousy and bitterness, Yui willingly turns against her former best friend.
Both girls, in the magical world, are treated as priestesses/prophetesses of the directions. Ancient China had four gods of the four directions: The White Tiger (West), Black Turtle (North), Blue Dragon (East) and the Red Peacock (South). Miaka becomes the priestess of the Red Peacock, while Yui becomes the priestess of the Blue Dragon. Each priestess is, under the right circumstances, able to summon her god and request the granting of 3 wishes.
Of course, Miaka's first impulse is to ask that she pass her high school entrance exams, so she immediately agrees to the role.
Each priestess has seven guardians, or "stars," whose gathering catalyzes the priestess' abilities; in the case of Miaka, the guardians are seven men and boys including one king, one money-grubber, one transvestite, a doctor, a student, a magic user, and a nutty clan leader. Miaka haplessly falls in love with the money-grubber, who is actually a handsome, caring young man called Tamahome. But, since this is a girls' comic, it shouldn't surprise anyone that not only does Tamahome also fall in love with her, but so does the king, the transvestite, and much later, one of the others. And, since this is a Japanese comic, it shouldn't surprise anyone that several of these characters die through the course of the story.
The series, while dipped heavily in romance and even something approaching outright sex (and, thankfully, large quantities of refreshing humor), largely follow Miaka's adventures as she seeks to find (first) all seven "stars," and (next) a new means of summoning the red Peacock god. (The plots, by the way, are interesting and done very well, if not always completely original.) Meanwhile, Miaka's friend Yui grows more distrustful and more bitter, and interferes with Miaka and especially Miaka's relationship with Tamahome. Despite this, Miaka still cares deeply about Yui, and tries desperately to save her friend.
It is ultimately Yui who first "wins"; she calls upon the blue dragon. Though she is still hesitant to kill her former friend, her bitterness and jealousy win, and she ultimately nearly brings her own world --- modern day reality --- ruin.
Of course, Miaka manages to win in the very end, but that's to be expected. The ending is a tad on the grandiose and moralistic side, but is still handled fairly well.
However, the author, despite what looks like clear intentions to end the series (at the end of book 13) appears to have been somehow convinced to continue the series. Thus, Fushigi Yuugi launches into a new story, with a different long-term goal.
Unfortunately, to write much about the second story gives away a lot about the first. Suffice it to say that Miaka's goal in Part II is to save the life of Tamahome, to help the red Peacock, and of course to save both worlds --- her own and the magical ancient China. The antagonist this time is not the servants of the blue dragon, but an evil entity who was theoretically part of the reason the blue dragon's side was so nasty.
All of the seven stars --- even the dead ones --- make a return appearance in Part II, and in fact some of them are given more depth. The reader learns much more about them: their pasts, weaknesses, and strengths.
Even more than the first section, Part II plays upon the theme of redemption and personal growth. Past mistakes are righted, the fallen are rescued, and internal doubts, failings and fears are confronted and at last overcome. The treatment of these themes, while somewhat heavy-handed (moreso than all but the end of Part I), is still laced with Yuu Watase's humor. The characters hold their own, and this keeps the story believable and interesting.
The flaws with Part II are somewhat irritating, but reveal incredible amounts of information about Japanese culture. For example, if you knew that a deadly and heartless enemy was controlling the minds and bodies of people at your school, would you go there? Yet Miaka does precisely this over and over again, with the dedication of someone who believes that graduating is her future and life. This attitude is not too surprising, given that Part I left her with the will to pass her high school entrance tests, but it does get a little ridiculous (I mean, why WOULD anyone keep going to a place one knows is run by the enemy?!). Near the very end, oddly negating what she learned in Part I, Miaka at last decides she can quit school --- but this is because she believes she'll marry Tamahome, and then what use is a high school education...? ...(sigh) ... As I said, the attitudes reveal a lot about Japanese psychology and beliefs.
In summary, Fushigi Yuugi is a good, solid, entertaining read --- if you can stomach many characteristic girls' manga elements, such as a lot of men falling for Miaka, and a lot of romantic scenes and declarations of undying love that one would normally never see in a year's worth of boys' comics. The humor and good plot work keep the story going, and even the moral messages manage to fit right in with astounding success. Yuu Watase somehow managed to pull it off.