YuYu Hakusho, Fuji TV
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This series is divided into, as far as I know, four sagas (or seasons, if you prefer). The first is the Spirit Detective Saga, which includes a number of story arcs. It all begins when 14-year-old Yusuke Urameshi, a juvenile delinquent, uncharacteristically tries to save a young boy from being hit by a car. Yusuke ends up being killed by the car himself, but the spirit world isn't ready for him, his action having been so unexpected. Botan (one of many soul-ferrying girls, referred to in the English version as "grim reapers") brings him to spirit world, which is run by Koenma (a toddler, who will sometimes take a teenage form) for his father, King Yama. Koenma gives Yusuke a chance to return to life if he passes a test (basically he's given an egg which will hatch into either a spirit beast which will protect him or one which will devour his soul, depending on how he behaves until it hatches). Botan will help him with this ordeal, though he can't directly contact anyone living. He does, however, manage to get some help from a couple of people who are mourning his death. His single mother, Atsuko, unfortunately, isn't much help; she's very irresponsible and drinks too much. But of more help is his friend (and potential love interest) Kayko Yukimura, who was in fact Yusuke's only friend. (I think "Kayko" is just the American spelling, in the Japanese version I believe it's Keiko. I must say this seems terribly odd to me, that they'd change it, because I've heard of plenty of Keikos, but this is the only one I've ever seen spelled Kayko. So what's the point?) She always nagged him, mainly about the fact that he hardly ever came to school. Truancy was of course the least of Yusuke's shortcomings. He was always getting into fights (and always won), and had a generally bad attitude toward everyone, including Kayko most of the time. His chief rival was Kazuma Kuwabara, who's probably the second-toughest punk at Sarayaski Junior High (though he considers himself the toughest, despite constantly losing fights to Yusuke). He mourns Yusuke's death, having lost basically his only worthy rival. He also has a strong spiritual awareness, and will be of some help to Yusuke in his attempt to return to life. However, in the end, Yusuke essentially fails the test, sacrificing the egg to save Kayko from a fire. But Koenma brings him back to life anyway. Then begins the second arc of the first saga, in which Yusuke becomes a spirit detective, working for Koenma, and assisted by Botan. He also acquires a power called a "spirit gun," which means he can fire spirit energy from his finger. His first assignment is to retrieve powerful artifacts stolen from spirit world by three demons: Gouki, Kurama, and Hiei.
In the next story arc, Yusuke competes in a tournament to determine who will become the apprentice of a powerful and grumpy old psychic named Genkai. One of the competitors is Kuwabara. Another of them is a demon named Rando, who has been gaining many powers for many years, and now seeks to learn Genkai's spirit wave technique. However, no one knows which of the competitors is actually Rando, who is in disguise. So Yusuke has to figure it out and defeat him. Yusuke eventually wins the tournament, so Genkai spends the next 6 months training him, but we don't see any of that, the time all passes between two episodes.
For his next assignment, Yusuke has to fight four powerful Saint Beasts who want to take over the mortal world, while Botan stays on Earth to battle the Makai insects the Saint Beasts have sent, which are infecting humans and turning them into zombies. Joining Yusuke in his battles are Kuwabara, Kurama, and Hiei. Those three will end up working with Yusuke alot throughout the series, and we'll learn more about them, including Kurama and Hiei's pasts. Kurama's actually a pretty good guy, though he wasn't always the person he is now... Hiei isn't that great, he's always only interested in... well, his own interests. But he does come to respect Yusuke, and their goals are sometimes similar. I should also mention that throughout the series, Yusuke, Kuwabara, Hiei, and Kurama will constantly increase their powers to face ever fiercer threats.
In the next arc, we meet Kuwabara's older sister Shizuru, who also has a strong spirit awareness, and also always nags Kuwabara to study more. Meanwhile, Kuwabara again joins Yusuke on his next mission, to rescue an ice apparition named Yukina from some gangster who's kidnapped her because her tears are valuable jewels. And we learn that Yukina is Hiei's sister, and she wants to find her long-lost brother. However, Hiei doesn't want her to find out it's him. We'll learn more about all this in the fourth saga. Meanwhile, Kuwabara falls in love with Yukina, but she always just seems to think he's joking around. Anyway, in the course of the rescue mission, Yusuke and Kuwabara easily plow through all the demons working for the mobster, except two: the Toguro brothers. Actually it's mainly just the younger brother (though the elder Toguro brother shows up again in the second and third sagas). After a fierce battle, the heroes win, just barely, and free Yukina.
But it later turns out Toguro (the younger) let them win, which begins the second saga, the Dark Tournament saga. Toguro gets them to enter a tournament of demons, competing in teams of five. Joining Yusuke and Kuwabara are Kurama and Hiei, as well as a mysterious masked fighter (though how anyone managed to not know who it was is beyond me). Kayko, Shizuru, Yukina, Botan, Koenma (sometimes in teenager form), and the main Ogre who works for Koenma, all attend the tournament as spectators. There's also a rich guy named Sakyo, who's betting on Toguro's team (which of course includes both Toguro brothers), and is Toguro's employer, and turns out to have the tournament committee in his pocket. He's pretty evil, but Shizuru has a thing for him anyway. I should also mention that the tournament is refereed/announced by a catgirl named Koto, and later a fish girl named Juri (both of whom we kinda like). Anyway, Yusuke eventually has to get more training from Genkai, making him more powerful than ever. Finally, he has to face Toguro at his heretofore unseen full power. Oh, and Yusuke's spirit egg finally hatches (I didn't see that one coming), and what emerges is a little furby-looking thing named Puu, who is spiritually linked to Yusuke. There are lots of surprises involving both Genkai and Toguro in this saga, and lots of great fights of course.
Next comes the Chapter Black saga, in which it turns out that a tunnel is being opened between the human world and demon world. Spirit world had long ago set up a barrier between the human and demon worlds, to keep the most powerful demons out of human world. Now that the barrier has been weakened, some humans are gaining new powers, and some of them will help Yusuke and his friends, while others (as well as the elder Toguro) help the latest enemy, Shinobu Sensui, who was spirit detective before Yusuke. Until he saw a video called "Chapter Black," which shows all the worst things humanity has ever done, which made him unable to continue defending humanity against demons. Yep, plenty more big fights for our heroes, and plenty more revelations, about Sensui and about Yusuke. Eventually the fight moves from human world to demon world. And of course, Yusuke once again discovers even more power within himself than he ever knew he had.
The fourth and final saga is the Three Kings saga, in which there's supposed to be a power struggle between the three kings of demon world, each of whom recruits someone to come train, become more powerful, and fight for them in the coming struggle. Yusuke is called by Raizen, Kurama by Yomi, and Hiei by Mukuro. We learn alot more about the pasts of Kurama and Hiei. And... the war never happens, because after Raizen dies, Yusuke proposes a tournament which all demons are free to enter, and whoever wins becomes king of all demon world. And a new tournament will be held every few years. So... I'm not going to say who wins the first tournament, but there are of course plenty of great battles. And after it's all over, we get to see all our old friends back in human world. The end.
...Although there are a couple of movies and some OVAs. I finally watched the second movie, Poltergeist Report (which I think has sometimes been used as a subtitle of the series itself), but I don't know if I'll ever see anything else. Probably none of this is canonical with the series, anyway....