Korosensei vs. Saiki Kusuo ~Showdown in Iruma~

Korosensei vs. Saiki Kusuo ~Showdown in Iruma~ (殺せんせーvs斉木楠雄 ~入間市最終決戦~, Korosensei vs Saiki Kusuo ~Iruma-shi saishū kessen~) is an extra chapter of the Assassination Classroom manga. Originally published in the Autumn 2012 issue of the Jump NEXT! magazine, it is a crossover between Assassination Classroom and fellow Shōnen Jump manga series The Disaster of Kusuo Saiki (斉木楠雄のΨ難, Saiki Kusuo no Sainan).

The creators of both manga series (Yūsei Matsui and Shūichi Asō) hail from Iruma, such is the theme of the chapter.

Short summary
Kusuo Saiki appears inside the Mitsui Outlet Park in Iruma to get his monthly irumanjū, a specialty dessert of Iruma. Just as he's about to grab the last piece, Korosensei appears to snatch it away from him. Saiki, however, notes his unusually fast speed and craftiness and vows to snatch it back. He replaces the manjū on Korosensei's hand with a trading card of local actor David Itō, then teleports away to the Sakurayama Observatory. Korosensei manages to follow him, and after realizing the two of them are alike, Saiki resolves to split the manjū between them.

Long summary
"Iruma. Located in the southwestern part of Saitama prefecture, it is a city known as the chief producer of Sayama tea."

Inside the Mitsui Outlet Park, Kusuo Saiki introduces himself as a PSI user. He explains that the reason he came to Iruma is to pick up a specialty, the irumanjū. He explains that although the name sounds ridiculous, the confection has a nice balance between the sweet bean paste made from Dainagon Azuki beans and the dough made from Sayama tea. He explains this as his reason to ride the red arrow express on the Seibu Ikebukuro Line once a month and come to Iruma and buy them. He realizes there's only one piece of manjū left and is about to take it when suddenly, a tentacle appears under his hand. He looks up to find Korosensei in disguise and wonders who he is, then looks back at Korosensei's sweating tentacle. Korosensei notes that the boy with antenna on his head was here for the irumanjū too. He then claims the manjū for himself, being a second faster, and laughs at him. Seeing Korosensei walk towards the cashier, Saiki is confused, but re-affirms that the confection was supposed to be his. He recalls his hand was already on the manjū, which then got snatched away at a very fast speed. He then recalls that the alien manicured his nails, possibly due to guilt, and finally replaced his hand below, all in just 0.2 seconds. He claims that with his speed, all he could do was watch as the alien beautified his nails. He notes his appearance and conduct are clearly of a monster's, but vows to take the manjū back from him anyways. The cashier (feeling awkward inside) greets Korosensei as he prepares to check out the manjū, claiming Shūichi is looking forward to eating it. The cashier points that he's no longer holding the last manjū, but instead, a trading card of local actor David Itō. Korosensei is shocked and turns to see the boy with antenna carrying the manjū away and smiling back at him. Korosensei starts to run after him. Saiki becomes annoyed at his persistence for just a piece or two of manjū and decides to disappear from him through teleportation, allowing him to move directly from one place to another in an instant, regardless of the alien's speed. Korosensei is left outside the store looking left and right for him.

At a balcony overlooking the city, Saiki is finally alone from the alien. However, Korosensei, now out of disguise, immediately re-appears behind him, claiming it only takes him three seconds to get here from the outlet store. Korosensei notes that they're now inside the Sakurayama Observatory, to which he claims has the best view to eat the manjū. Saiki is surprised at the alien sharing his thought. Korosensei remarks of the natural view of the hills to the west and the growth of commercial establishments to the east and south. He claims that PSI users and super creatures alike have places where they belong and asks the boy he thinks this place is one too. Saiki starts to believe the alien likes Iruma a lot, only to read his mind and realize that he's thinking of nothing more than the irumanjū, with the word literally all over his head. Reflecting that he's the same too, Saiki gives half of his manjū to the alien, surprising Korosensei to predict the boy saw right into his heart.

Characters in order of appearance

 * 1) Kusuo Saiki
 * 2) Korosensei

Trivia

 * On the chapter cover, Korosensei's name is changed from Koro Sensei in the Jump NEXT! publication to Koro Teacher in the volume 2 publication.