Back in Time and a Giant Wall of Destructionis an episode from season six.
Natalie goes to the Cartoon House to talk to Ghetsis for an unrecalled reason. He creates a giant "reset wave" that causes everyone to forget everything they learned from the Cartoon House by going back in time. Once people figure out what it does, the mass runs for the huge wall.
When the wall was closing in on them, they tried using black holes created by Natalie to fix it, but in the end everyone was 'reset' except for Dark Pit, who escaped using the Reincarnation. However, Kali and Julian create a blue wave that returns them to normal shortly after.
- On the extras in the disk for season six, there is an alternate ending for this episode where the entire shows starts over. Apparently, after many years the show is exactly where it had started, even showing Natalie running to Ghetsis, creating a loop in time.
"Natalie accidentally has Ghetsis create a reset wave."