Phineas, Ferb, Jessie, and Perry were sitting on the couch late at night watching an old alien horror movie. Jessie had known the planet the aliens in the movie lived on; she had a summer home there. Candace came in and decided to watch it with them. By morning, they had all fallen asleep, except for Candace. She was kind of crazy though so it's all good.
Candace had gone upstairs to tell her siblings how good the movie was, but she heard some strange language she didn't understand coming from Ferb. Later, she saw what looked like Ferb ripping the head off of... Ferb... Somehow it made sense to her. Candace had suspected that Ferb is in fact an alien.
Candace ran to Phineas' room where he and Jessie were pelting each other with styrofoam bricks (It was a common pastime in her culture). Candace explained to them that she thought Ferb was an alien.
"But.. Candace... Ferb is part alien. So am I..." Jessie explained to her crazy step sister.
"...oh right. I forgot. Carry on!" Candace said and skipped out of the room humming a tune. She was worried for nothing. Ferb was an alien the whole time!