The Forbidden Act is the eighth chapter in the Vampire Knight manga.
A flashback shows a young Zero talking to his sensei, Toga Yagari, who is resting under a tree. Zero apologizes for returning to his sensei after being told to run away, as his actions caused Yagari to lose an eye. Yagari scolds Zero for being upset. Zero asks why vampires hurt humans and Yagari explains that vampires are beasts who live on natural instinct. He adds that they are enemies of vampire hunters, who live only to kill vampires.
Zero winces at the bullet wound he received. Yagari explains that a spell was imbued on the bullet he used, which was made to kill vampires. He points his gun at Zero but Yuki protects him. Yagari recognizes Yuki as Kaien Cross's daughter and says that her wanting to protect Zero is not enough of a reason to spare him. But Yuki says that she won't let Zero die, causing Yagari to ask if Zero hasn't reduced to a Level E vampire yet. Kaien then arrives and stops Yagari, claiming that his actions are the reason why he hates vampire hunters. He tends to Zero and Yuki, pulling them out of the water, can Yuki asks who Yagari is. The vampire introduces himself as Zero's former teacher, which Zero confirms. Yagari asks why Yuki would protect Zero, a vampire, when he himself is a vampire hunter. Yuki ponders his words.
That night in her room, she thinks quietly about how Zero is talking with Kaien and Yagari whilst she is not. She remembers Kaien telling her not to worry but remembers Yagari saying he'll kill Zero. Yuki then thinks about Zero's blood lust and how ineffective the tablets are. Her roommate Sayori Wakaba awakens and asks if Yuki is all right, as she doesn't seem herself lately. Yuki doesn't reply and instead thinks about how useless she is.
The next morning, she sees that Zero isn't at school. She sees Yagari walking in the hallways and demands to know where her friend is and is told that he is being quarantined. Yagari says it's to ensure that any violent attacks to possible come would not be in effect. Yuki asks if he's still okay and asks where he is. Yagari simply ignores her question and leaves, saying that Kaien has instructed him to act as the Night Class' new teacher, much to Yuki's surprise.
Yagari closes the door to the Night Class' homeroom. He introduces himself to the students there. Rima Toya recognizes him as the current top Vampire Hunter and Hanabusa Aido realizes he fired the gunshot from the night before. Yagari explains that he's actually a qualified teacher with a proper license. Kaname Kuran notes how he was supposed to be taking a break and asks if he's checking out the Night Class, or if he's actually aiming to kill a vampire there. Yagari simply greets Kaname and threatens to kill him if he doesn't listen to his lectures. After the lesson is over, Yagari goes outside and sees Yuki in the hallway, having waited the entire time. Yuki says that she was worried for him and that she doesn't want to neglect her duties as a Guardian. Yagari tells her that no matter what, she can't do anything, but nonetheless decides to tell her about Zero's location.
Yuki then leaves and finds Zero in the Headmaster's private guest room. He tells her to leave but she refuses to. She asks about Yagari and Zero explains that he took care of him and his younger brother before when his parents were away. He then explains the incident that cost Yagari his eye: a female Ex-human vampire attacked Zero and Yagari protected his student, but in the process lost an eye. Zero explains that the vampire always bared her fangs in the fight and that caused Zero to develop a fear of vampires. Yuki tells Zero that he's giving up on everything, including himself. She says that he gave her a gun and that he shouldn't give up on her. Zero refuses to look at Yuki, upsetting her. She then takes him away to a bathroom and tells him to finish drinking her blood.
Takuma Ichijo sees Kaname outside in the hallways and asks if he's coming back to class. Kaname says that he doesn't want to listen to Yagari's teachings. Takuma notes how Yuki has left and Kaname says that she's very kind-hearted.
Zero questions Yuki's actions but she says that it's for the best, because even if it's only for a little while, her blood can make Zero feel better again. Although reluctant, Zero gives in and drinks the blood from Yuki's neck. Yuki says that what they're doing is the most unforgivable act.