Killing Time in Homeroom

Killing Time in Homeroom (暗殺の時間, Ansatsu no jikan) is chapter 1 of the Assassination Classroom manga.

VOMIC adaptations of the chapter were aired as weekly segments of the Japanese variety show Sakiyomi Jan Bang! throughout January 2013 and released online throughout February 2013 on Shueisha's VOMIC website and on July 8, 2013 on YouTube via the official Shōnen Jump account.

Short summary
A new alien teacher begins his homeroom class at Kunugigaoka Junior High School with students firing BB guns at him. The students are distraught to find the teacher unharmed.

It is revealed that just before teaching the class, the alien managed to carve the moon to become a permanent crescent moon. With his superhuman powers, he had effectively taken over the world and plotted to destroy the Earth in a year. However, he also promised the governments of the world to teach a certain class of students in the meantime without harming the students. At the same time, the Ministry of Defense, under Karasuma, promised the students a 10 billion yen reward if they can assassinate their teacher before he destroys the Earth.

At lunchtime, the alien teacher zooms off to Sichuan province in China to have some mapo doufu. Back at school, Terasaka and his friends pressure Nagisa with their own method to kill the alien teacher for money.

After lunch, Nagisa approaches the alien teacher to explode a BB pellet grenade on him successfully, as planned by Terasaka. However, the alien teacher survives anyways. He collects the nameplates of all the students' houses and threatens to kill everyone's family relatives. In the end, he praises Nagisa for his efforts but scolds Terasaka and his friends for disregarding Nagisa's safety. Kayano finally thinks of a name for their teacher, Korosensei, the unkillable teacher.

Long summary
During homeroom, in class 3-E, all the students inside were sitting in their seats nervously as the octopus-like alien creature places down a purple-blue book labelled 'Attendance' onto a table. He asks for whoever on day duty to issure the commands, and Nagisa issues a loud 'STAND!' shout, everyone in the classroom getting up with guns and shooting the alien-teacher.

The alien grins and dodges all of the bullets, calmly taking attendance of the class. Though this shocks everyone, they still went along with the attendance call. When the list was finished, the alien was happy to find that no one was absent, and by now the shooting has stopped already. The alien informs that their killing tactics are way too simple and if they wanted to kill him they need to devise a better plan. The class gets enraged, claiming that the bullets were only BB Gun pellets- and that the alien can probably endure the pain if he would feel any.

Doing a demonstration of what the BB Gun pellets can do to his body, the alien shoots his right arm off, shocking everyone, then later regenerates it. He then tells everyone to clean up the class from the BB Gun pellets. While teaching a class, as he asks Kimura a question (to which the latter answers correctly), Nagisa and Kayano tries to remember how it all began, triggering a flashback.

In the past, there were reports of the moon's 70% disappearing mysteriously, leaving only a crescent moon visible for the rest of everyone's lives. It then skips to the 3-E classroom, the alien teacher introducing himself as 'the one who blew up the moon' and their 'new teacher'. By the looks of the students at that time, it seems that no one believes the creature yet.

Karasuma, a guard from the Ministry of Defense, then tells the class their assigned mission - to kill the octopus-like creature. As everyone were shocked, Karasuma starts to explain everything that happened between the monster and them so far. He tells them that the creature plans to destroy Earth next year, at March. Karasuma takes out a blade and tries to stab the monster repeatedly, only to have him dodge every single move while hilariously in the process of trimming Karasuma's eyebrows. He reveals that the maximum speed of the octopus-creature was Mach 20, and continues explaining their objective.

At lunch, the creature uses it's Mach 20 speed to travel to China to buy mapo doufu. While the creature is away Terasaka with his friends Sōsuke and Yoshida question Nagisa if he has figured out what each face colour of the octopus-creature's face means in the yard. Terasaka rudely cut Nagisa off as he explains what he has learnt and tells Nagisa that he simply has to stab the creature when it's guard is down. Terasaka also reminds Nagisa that class E is referred to as the "end class", the class for the school's losers and rejects when he is reluctant to go through with the plan. Nagisa is then given a small bag by Terasaka and left to reminisce about how he was treated after his peers found out he was sent to class 3-E.

The creature returns with a missile as Nagisa quickly hides the bag he was given behind his back. Nagisa comments on how tough it must be to be everyone's target and the creature replies with how he believes it just proves how powerful he is. Nagisa realises that the creature will never understand how he feels and will never notice him. He returns to the school building with a willingness to go through with Terasaka's plan.

In fifth period, the creature asks the class to write a free-poem that finishes with the word tentacles. After it tells the student that once they have finished they can leave Kayano asks if the creature has a name. It says that it doesn't and tells Kayano that she can come up with one after she has finished her poem.

Nagisa then takes action moving forward towards the octopus creature to attempt an assassination after noticing its face has changed color to a light pink. This pink face signifies that the creature is feeling more relaxed and Nagisa assumes that this is probably when the creature has its guard lowered the most.

After taking a swing at the creature with an anti-it knife. The creature starts to chide Nagisa. Nagisa proceeds to calmly hug the creature revealing a grenade that is hanging around his neck. Terasaka then proceeds to blow it up using a remote control. Terasaka proceeds to explain how the grenade is stuffed with special bb's and gunpowder he also says how he'll pay off Nagisa’s medical bill with the 10 billion of reward money.

As Terasaka approaches Nagisa he is confused by the filmy stuff covering Nagisa. From up on the roof behind him the creature reveals that it sheds its skin once a month or so. Its color has changed to pitch black and it is furious at Terasaka, Yoshida and Muramatsu.

The creature then flies at Mach 20 collecting the nameplates from every students home and reminds the students that it only promised not to hurt the students and that everyone else is fair game.The students realise that the only way to keep everyone safe was to kill it after it threatened to kill everyone in the world but them.

The creature then proceeds to praise the plan and then Nagisa as his movements were natural during the attempt before scolding the rest of the group for not considering Nagisa's safety and for taking advantage of him. It procliams that he wants the students to assassinate him with a method that will bring a smile to their faces and that they can take pride in.

The creature then asks Nagisa what he needs to do to prevent the creature from destroying they world. Nagisa responds that he'll have to kill it before that happens. This causes green stripes to appear on the creatures face as it says that he'll let Nagisa go home early if he kills it right now. Kayano then comes up with a name for the creature, Korosensei, which is a pun on korosenai meaning can't be killed and sensei meaning teacher.