Twelve-year old Aoyagi Ritsuka lost his only kin when his brother, Aoyagi Seimei, was killed under mysterious circumstances. One day, he meets Wagatsuma Soubi, who claims to know his brother. It turns out that Soubi and Seimei used to be a fighting pair, whereby Soubi was the "Fighting Machine" and Seimei was the "Sacrifice". Now that Seimei is gone, Soubi 'belongs' to Ritsuka who will become his new "Sacrifice". After learning that Seimei was killed by an organisation known as the "Seventh Moon", Ritsuka decides to join forces with Soubi and investigate the truth behind his brother's death.
Ritsuka is still troubled by the appearance of Soubi. However, when he finds Seimei's will, everything's going to change. Then again, Yuiko is still trying hard to become Ritsuka's friend. But being friends is one of the last things on Ritsuka's list, especially when the Breathless team returns to get revenge.
Ritsuka finally cracks the code the Seven Moon members give him. It leades him to a internet computer game, where he meets up with one of the members. However, later on, he finds everyone dying, and Soubi admits he killed them all. What can Ritsuka do now?
Ritsuka refuses to join any club activities because it clashes with his therapist’s appointments. While he worries ever more about forgetting his past, she offers to take him out for a “date” to get to know him a bit better, where he accidentally reveals the extent of his mother’s abuse of him. Meanwhile, Ritsuka’s teacher attempts to make another home visit, but gets accosted by the latest pair of enemies, codenamed “Zero” They take her captive and Soubi intervenes and takes them on without Ritsuka present as his sacrifice.
A meeting with opposing teammember Yamato gives Ritsuka a glimps of a way to beat the almost unbeatable 'Zero' team. Can he figure out how to turn that weakness into an advantage fro he and Soubi in time to defeat them?
After fighting the new 'Zero' pair, Soubi is badly injured. Ritsuka tries all he can to help, but in the end, is forced to rely on Natsuo and Youji. Can he trust these ex-Seven Moon members?
After a brief argument, Ritsuka tests Soubi's loyalty by ordering him to bring him a picture of the rare Iromote cat. This turns out to be a big mistake, especially when the Breathless team returns again. Without Soubi to help him fight, can Ritsuka make it out on his own?
Soubi's battel with the Zeros continue. But however, when it's finally over, he comes down with really serious injuries. Ritsuka grows suspicious of Soubi's constant lies, and finally decides to do something about them. Meanwhile, Shinonome is still haunted by Soubi's words to her.
A new Zero is in town. In order to keep Ritsuka out of danger once again, Soubi fights the team alone. But this time, he's probably met his match. Ritsuka, felling that there's something Soubi's hiding from him, attempts to find him. Can he make it in time?
During a trip to Yokohama, there seems to be a huge mixup on what happened. Many of the characters claim they saw Yuiko and Ritsuka 'kissing' on the Ferris wheel ride, but did that really happen. And will this affect Ritsuka's relationship with Soubi?
By the mystery of his brother’s death, and the existence of Soubi, Ritsuka was spending a night without sleeping. Then, another sacrifice and fighting machine came to him. They were Kinta and Gin. They showed him an envelope. They said there was what Ritsuka wanted to know inside. What's inside the envelope? Is it truely what Ritsuka wants to know?
Parent-teacher meeting is coming up. Unfortunately, that's not the only thing troubling Ritsuka. Soubi hasn't appeared to pick him up during the last couple of days. What's going on?
Still recovering from the recent death of his late brother Seimei, Aoyagai Ritsuka is transfered into a new school. After a terrible day, he meets up with the mysterious Soubi, who claims to be Seimei's friend.