Present day:
Nagi Souchirou and Bob Makihara are two delinquents who arrive at their new high school, at first to merely assert their "rule" over the school. Gradually they get pulled into Natsume Maya's personal war, starting perhaps at about the time Ryuzaki arrives.
Past:
Natsume Shin, who is dead in the present day, used to be the president of the Enforcement Group, which basically oversees and keeps all the fighting the school is famous for from going out of bounds. People call those days the "Golden Age" of the school (forgot the exact name of the school, but who cares?)
Anyway, Shin also heads a group called Katana during the beginning of the past arc, along with Tawara Bunshichi, a fellow junior. During that time, Maya and Takayanagi Mitsuomi are new freshmen at the school. Lately it turns out that someone's been beating up Katana members. The perpetrator turns out to be Maya, but this isn't revealed until later.
Anyway, what's going on behind the scenes is that master Dougen (forgot his full name. Takayanagi Dougen?) wants to create the "ultimate warrior" who he intends to be Mitsuomi. To create this ultimate warrior, the ultimate warrior must first have an absolute darkness to overthrow, to quote Dougen, and that darkness is Natsume Shin and his Dragon's Eye, which allows him to know exactly what his opponent will do before they actually do it. It also puts him in an uncontrollable murderous rage as well.
This ability was first unlocked when the magical katana known as "Reika" was first handed down to Maya for safe-keeping. Because she had no innate powers like the Dragon's Eye, Reika could not affect or corrupt her in any way. She was told to not let anyone touch it, but she lets Shin touch it anyway. Shin goes nuts and has to be kept in a cell---from that point on his sanity is more or less doomed as he sees apparitions of dead people inside his cell on and off. Later on, he murders his parents with Reika. The katana has the power to "trigger" the Dragon's Eye ability inside Shin and is the source of much of the suffering that goes on in the manga.
Now, to keep an eye on Shin, Dougen and his organization employs Mana to act as his "girlfriend." On a special note, Mana is an expert with acupuncture arts. At one point, she seduces Mitsuomi. Her reason for this can be summed up with her words, "Are you afraid of Shin too?" Shin probably knew for a long time that her real job was to keep tabs on him, not as a genuine girlfriend.
Anyway, to go a bit further on, the Enforcement battles are approaching, which is held annually to determine who will comprise the Enforcement group. At least five members must be in each team. And since the Katana group wearing down due to the constant Katana-hunting being done by Maya (a sort of rebellion against her brother, I suppose) Katana was scrapped and Shin proposed the creation of a brand new group for the Enforcement battles.
Hence forms the Jyuukenbu, made up of: Natsume Shin, Tawara Bunshichi, Natsume Maya, Takayanagi Mitsuomi, and Sugano (forgot his full name. Nickname, "Asshat-kun").
Fu'Chien, that arrogant Chinese dude, decides to take matters into his own instead of following Dougen's plan and moves in to draw Shin out. He doesn't get why anyone needs to fear Shin (an untrained teen from this something-something country, he says) So, he takes Maya hostage (stripping her in the process) during the Enforcement battles. Shin arrives and totally annihilates Fu'Chien and his group.
Isuzu (that knife-throwing girl who's really a super-fat chick who uses her spirit to hide all those knives within her fat) tries to stop Shin from going any more out of control but gets nailed to the floor with her own knives. As Shin is about to kill her, Mitsuomi enters the room and rushes toward Shin. Maya cries out Mitsuomi's name and regrets it, since Shin wants to get rid of everyone close to Maya (sister complex
mwahaha) So, Shin puts his hand to Mitsuomi's chest and blasts his heart with a load of chi.
What follows is that as a result, Mitsuomi only has a few more years to live, but his heart pumps more blood than usual. As a result for short periods of time he is capable of far more strength, speed, and reflexes than is normally possible. Hence during the final Enforcement battle, Mitsuomi totally wipes out the last opposing team within three minutes, after which he leaves the scene, finds Mana, and throws up a ton of blood. As Mitsuomi says, his life as a martial artist has ended (not actually dead yet, of course).
By this time Maya already loves Mitsuomi and Mitsuomi in return (I think). Mitsuomi comes to Maya's house to get her to explain everything about Shin and what had been going on. Maya's about to say something when she sees Shin behind Mitsuomi, holding a sword against Mitsuomi's back. Maya begs for Mitsuomi to go home. Mitsuomi sees Shin's reflection in Maya's teary eyes. Her tears tell the whole story to Mitsuomi, quoted from Mitsuomi, and Mitsuomi promises that he will beat Shin after two months.
Two months pass and Shin and Mitsuomi finally fight. Mitsuomi is totally beating the hell out of Shin, primarily cause Shin is resisting against the temptation to use the Dragon's Eye, sort of like resisting the temptation to use the Dark Side of the Force, in other words.
Shin's Dragon's Eye had been giving him this vision of a person who delivers death to Shin. For a long time Shin has had that vision, the last thing he sees before death being the fist of that person, the final punch delivered that will end his life.
Mitsuomi is obviously that person. Anyway, at one point Shin tries to take down Mitsuomi with him by trying to smash Mitsuomi's head against the floor with his elbow, but Mitsuomi knocks Shin off. You see Mitsuomi in that fighting stance, smiling, looking at Shin falling down towards the floor. That very scene is shown in the first volume (or the second) as well, except that Mitsuomi is all blacked out. A minor note, really.
Anyway, as Mitsuomi is closing in for the final hit, Mana stops him by getting between the two. Mitsuomi backs off, and Shin sees an opening for the first time in the battle. His Dragon's Eye manifests and he unleashes the Forged Needle Attack against Mitsuomi, THROUGH Mana. Mitsuomi is effectively KO'ed, while Mana has had about half of her midsection blown away.
Shin and Mana have this little tender moment together where Shin takes one of Mana's acupuncture needles and drives it into his ear and the two die together.
Fast forward back to Present Day:
Mitsuomi is now president of the Enforcement Group. Fighting in the school has changed from honorable to one in which anyone with overwhelming resources can win. Mitsuomi's younger brother (I forgot his first name. He will be referred to as Takayanagi for now ^_^) and the father have been run out of the family, and Mitsuomi has taken over the organization and company.
Anyway, Souchirou, Bob, Aya and Maya have a run-in with Mitsuomi and his Enforcement group, consisting of Isuzu, that staff guy, and that other girly-looking dude. Bob almost joins the group but does not after his girlfriend (forgot her name too) convinces him not to.
Souchirou and Mitsuomi, after their first round, almost go at it again when Bunshichi stops Mitsuomi by pouring water all over him. It looks, at this point, that the whole conflict is nothing but a lover's quarrel between Maya and Mitsuomi (to quote Bob's girlfriend). Bunshichi later on tells the past story to Bob and Souchirou.
*geh, my hands are getting tired now*
To really shorten what follows, Aya now has Reika. Souchirou can use the Forged Needle Attack. Bob feels as if the others are leaving his behind in power. The Jyuukenbu finds Asshat and brings him back into the group, along with that weird UFO-looking guy.
Group F, who were loosely working for Mitsuomi (but mostly just for themselves) will be fighting the enforcement battles. Mataza, one of F, kills off most of the Enmi family (those weird spirit, shrine maiden people). The heir escapes to the Natsume house, where the Jyuukenbu hang out most of the time.
Now, by volume ten, Souchirou has already fought one of F and is now fighting Mataza himself along with Aya, Takayanagi, Bob, and that little Enmi kid.
I'm sure I missed some, but I've covered the important stuff