Chapter 7

Toxin Time (毒の時間, Doku no jikan) is chapter 7 of the Ansatsu Kyōshitsu manga.

Short summary
Okuda attempts to deliver various poisonous chemicals directly to Korosensei. After a few head transformations and an unsuccessful assassination, Korosensei offers to help Okuda create a poison for him. The next day, Okuda delivers the poison, which turns out benefiting Korosensei from assassination by making his cells more fluid so that he can fit through small spaces. Okuda learns that it is not enough to perform a successful assassination by excelling in science, without mastering the art of language. Meanwhile, Karasuma receives word of a new assassin to be sent into the school.

Long summary
Korosensei concludes a chemistry class experiment to extract food coloring from candy and collects the leftover candy. The students note that he's taking their candy as snacks for himself and wonder why he still needs to live from paycheck to paycheck even though he's going to destroy the world. Okuda walks up to Korosensei with bottles of chemicals, telling him they have poison and to drink them. He easily notes of Okuda's straight forward assassination attempt, to which Okuda claims she's bad at surprise attacks but did her best to make them. The class is stunned at Okuda's blatant attempt, but Korosensei proceeds to drink them anyways. The first test tube Korosensei drinks causes him to grow horns around his head. He detects the chemical as sodium hydroxide and that it's harmful to a normal person but not to him. Korosensei then drinks the next test tube, which causes wings to grow out the back of his head. He detects thallium acetate, then proceeds to drink the last test tube. The last test tube returns his face to normal, but with a sustained straight face. As the students are puzzled by the pattern of transformations, Korosensei detects aqua regia. The students tell Korosensei his face is tiny like an emoticon, to which he tells the students not to hate assassinations even if they hate him. Korosensei expresses his concern for Okuda making toxins on her own and she apologizes. He then offers to work together with Okuda to research a poison to kill him after school and she accepts. Nagisa and Kayano are left stunned at Okuda's actions.

After school, while preparing the poison, Korosensei notes Okuda's excellent grades in science. Okuda claims she's no good at anything else even after dropping to class E, especially Japanese class. She explains she doesn't understand words or people's emotions well, but that it doesn't matter since math and chemical formulas have concrete solutions. After listening to Okuda, Korosensei gives her a procedure in preparing the solution as homework.

The next day, Kayano spots Okuda happily carrying the flask of toxin, confident that Korosensei has been supporting all along. Nagisa is stunned at Korosensei providing a manga for her to read about the proper storage methods of the poison. Korosensei enters the classroom and Okuda proceeds to pass him the flask of poison. Korosensei smiles and swiftly drinks the poison. He starts to transform and thanks Okuda for her medicine allowing him to evolve. Okuda and the rest of class are shocked to see Korosensei melt in the end. Korosensei explains the medicine Okuda gave him increases the fluidity of his cells. He slips inside Kataoka's desk and explains he can fit into any crack or crevice. Finally, he invites the class to try to kill him as he slides through the floor and ceiling of the classroom in his usual speed. Okuda finally realizes that Korosensei tricked her. Korosensei teaches her that deceiving people with language is necessary in assassinations, and that with naive honesty, no matter the quality of her poison, the target will take advantage of her. As an example, he asks Nagisa how he would give him poison. Nagisa responds by saying he'd put the poison into a juice he'd like and give it to him. Korosensei finally slips back into his outfit as the effects of the medicine wear off. He explains to Okuda that her talent in science can be useful to everyone, and that she should work on improving her language skills to be understandable and to better deliver poison. Okuda obeys his suggestion and Karma laughs a little at the previous attempt. As the next lesson starts, Nagisa concludes that there have still been no students able to endager his life.

Outside the school building, Karasuma answers a phone call from the chief of the Ministry of Defense. The chief claims the government never expected the students could kill Korosensei, and that they have to send a new assassin into the school even if it may endanger the students. Karasuma asks what kind of person the assassin is. The chief explains that this pro assassin has a proven track record with experience at 11 different jobs around the world. Meanwhile, this female assassin is seen licking her pistol on top of a naked man, having shot a bullet through his forehead.