This is a one-hour halloween special where Phineas and Ferb go to Isabella's house when Mrs. Garcia-Shapiro screamed.
They went down to the kitchen, but she was gone. Then a portal opened. They were sucked into it, and they landed in a place everyone called Halloweenville.
Everyone said it was always Halloween there. They had to find Mrs. Garcia-Shapiro and get back to the portal before it was too late!
Meanwhile, Dr. Doofenshmirtz and Perry accidentally locked themselves in the closet, and a monster from the portal goes to the Phineas and Ferb World and mistakenly goes to Doofenshmirtz Evil Incorporated and enters the closet door where Perry and Dr. Dooenshmirtz are, so now both Perry and Dr. Doofenshmirtz must work together (Dr. Doofenshmirtz will also trick Perry at the end, but Perry gets the last laugh) to get out of the closet, and defeat the monster.
Season: 9 Episode: 20