2. Vegeance Is Mine: Because Karai gets mutated and The Turtles don't know how to turn her back to normal.
3. Annhilation: Earth! Part 2: Just watch the ending.
4. Earth's Last Stand: Because the Fugitoid sacifaces himself to save Earth and stop The Triceratons.
5. The Invasion, Part 2: Leo gets badly injured/ambushed by Shredder and The Foot Clan, Master Splinter is almost killed by Shredder and thrown down a drain pipe, and New York is coquered by The Kraang.