The story revolves around Karma and Nagisa meeting at the old school building and talking about past events. Especially at the beginning and end of the movie there is new original animation about Karma and Nagisa. For the first half hour of the movie they talk about specific events that involved them during the first and second season. This is just the first half hour of the movie which means there's only a few scenes that can fit and most of the things that happen in the entire anime are left out. After this 30 minute "recap" the movie spends the rest of the time showing the last few episodes of the anime with some scenes left out. All in all that means this movie comes down to 30 minutes of random scenes followed by the ending of the anime with some original animation of Karma and Nagisa.
The characters are the exact same as in the anime since this is a recap. The animation, sound and voice acting are also the same since the animation from the anime was re-used. The original animation of Nagisa and Karma and Nagisa's narration of some scenes is also of the same level.
Overall you shouldn't watch this movie if you want a recap of assassination classroom. It only shows random scenes and then shows the ending. I think the main reason for watching this is seeing a bit more of Karma and Nagisa and the events they thought were important to them.
