If you tell me what you'd like to in the post, I can adjust the tone:

A professional assassin arrives, proving that Class 3-E isn't the only one after the 10-billion-yen bounty.

We see more of the group dynamic as they realize they must protect their "classroom" environment from outside interference. Why Watch the Dub?

(e.g., more about Nagisa or Koro-sensei)