Episode 38

Past Time is the thirty-eighth episode of the Assassination Classroom anime and the sixteenth episode of Season 2. It premiered in Japan on April 28, 2016.

Summary
The captive assassin and Aguri Yukimura continue to bond over their time conversing with each other. On the anniversary of their first meeting (March 13), the crescent moon came to be as a direct cause of Yanagisawa's anti-matter research. To avoid Earth meeting a similar fate in one year's time, Yanagisawa prepares to kill the assassin but the latter turns into a tentacled monster to escape the lab. However Aguri is killed by a hidden mine, and the assassin promises to carry on teaching her students for the remainder of his life. After Korosensei finishes his story, the class realise the heavy burden that is placed on them to kill their teacher.

Plot
Over the course of a year, The Reaper's body continued to be altered by the experiment while his relationship with Aguri continued to grow stronger. Meanwhile, an experiment testing the effects of the tentacle formula on a rat proved disastrous, causing the explosion that destroyed seventy percent of the moon. Learning that the same thing may happen to him in a year's time, The Reaper unleashed his new powers in an attempt to escape from Yanagisawa's facility. As Aguri tried to keep him from leaving, she was fatally wounded by one of the facility's traps, asking him to watch over Class 3-E in her place. Devastated by Aguri's death, The Reaper wished to be weak and flawed, and his tentacles transformed him into the kindhearted teacher Koro-sensei. Back in the present, as the students are troubled over the thought of having to assassinate the teacher they had grown to admire, Nagisa states his desire to find a way to save Koro-sensei's life.

Manga and Anime Differences

 * The self destruct device implanted inside Korosensei was omitted.
 * Korosensei engages Yanagisawa and his men in a hallway instead of a library.
 * Yanagisawa's men equip heavier armor than in the manga.
 * The anti-tentacle fence took on the form of a sheet of acrylic glass instead of a gridded fence.
 * Yanagisawa's eye was damaged by a metal tube instead of a shard of the anti-tentacle fence.
 * When the final semester begins, Korosensei meets the students in the classroom rather than outside the school.

Trivia

 * The opening song Bye Bye Yesterday is not featured in this episode, instead only showing the title card.