Before Brian and Quagmire were at zardy's maze, Stewie tried following them. But he couldn't keep up, running out of energy and eventually passing out. When he woke up, he found himself meeting against the corruption, however, BF and GF found him and saved him from getting corrupted. They had to fight in order to save Pico. But Stewie noticed something odd about GF.
Let me know your predictions on what happened at the end of the song in the comments. Edit: I actually Messed up on the Title and put "Finale" instead of Final.
