Ansatsu Kyoushitsu

Synopsis

Koro-sensei makes Class 3-E their own private pool, which alerts the students to one of his biggest weaknesses; his tentacles becoming bloated when soaked in water. Meanwhile, class bully Ryouma Terasaka is becoming increasingly irritated with everyone getting along with Koro-sensei and agrees to collaborate with Shiro and Itona in their assassination plan. After spreading a gas in the classroom and adding something to the pool the previous night, Terasaka follows Shiro's plan and uses a transmitter which he was told would signal him and Itona; however, the transmitter actually sets off an explosion which puts all the students in the pool in danger, forcing Koro-sensei to get his tentacles soaked with contaminated water in order to rescue them before they are swept away, leaving him at the mercy of Itona. With a few students still in danger, Terasaka, feeling guilt for his actions and anger for being tricked, follows a plan concocted by Karma, using his gas-covered shirt to infect Itona, allowing Koro-sensei to rescue the remaining students whilst the others use the contaminated water to put a stop to Itona's tentacles, forcing him and Shiro to withdraw yet again.

(Source: Wikipedia)