The "Mysterious Force" is a concept believed by Candace Flynn to be the reason of why Phineas Flynn and Ferb Fletcher's Big Ideas disappear instantly before she gets to show them to their mother, Linda Flynn-Fletcher.
Stacy is initially skeptic but becomes a believer when a portal appears in the back yard and it "eats" Candace. She then builds an altar to it to try and make up for her skepticism and bring Candace back.
When Candace a Stacy find themselves on the run after their Tree House Robots are destroyed by the never ending 2nd Dimension army of Norm Bots, Candace admits to no longer believing there is a mysterious force behind her brothers inventions disappearing. But she does still believe that her attempting to bust them leads to their big ideas vanishing as she soon after tries to bust Phineas and Ferb with the sole hope that the attempt will make everything out of the ordinary that is associated with their adventure disappear.
- "Phineas and Ferb The Movie: Across the 2nd Dimension" (mentioned)