第 21 集: Koro-sensei 使用綽號讓 Gastro 感到困惑，而 Takaoka 則揭露了他涉及 Kayano 和解毒劑的計劃。

With Gastro having a keen sense for determining an opponent's location, Koro-sensei gives instructions to the students using varying nicknames in order to confuse Gastro, giving Chiba and Hayami the encouragement to rise above their failed assassination and get the drop on Gastro. As the students approach the top floor, Koro-sensei deduces the mastermind isn't an assassin, while Nagisa learns Terasaka has been hiding the fact that he too was infected with the virus. Arriving on the top floor, the group discover the mastermind to be none other than Takaoka, who had planned to use Kayano in a plot to kill Koro-sensei when he emerges from his crystallized form. Wanting revenge for the humiliation he faced before, Takaoka calls up Nagisa for a rematch to their earlier knife fight. Despite Nagisa following Takaoka's demand to get on his knees and declare he's a coward, Takaoka blows up the antidote in front of the class, making Nagisa, who thinks everyone will die, pick up a knife in preparation to kill him. (Source: Wikipedia)

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