In "That's the Spirit:" Phineas and friends, while trick-or-treating, come across a haunted house in the neighbor that hasn't been there before. Meanwhile, Dr. Doofenshmirtz, having declared war on grass, gives a cow a piece of his mind, but the process unforeseeably gives him a kind of mad cow ailment in return. In "The Curse of Candace:" having just seen "Early Evening" at the movie theater and suffering a bat attack in its lobby, Candace becomes convinced (thanks to coming across several trick devices developed by Phineas and Ferb) that she's now a vampire. Meanwhile, Dr. Doofenshmirtz thinks he'll have a better time taking over Danville if he transforms it back to an earlier age.
—statmanjeff