As Candace Flynn's day spirals into chaos, she suddenly finds herself on an unexpected journey inside a mysterious space pod, accompanied by her friend Vanessa Doofenshmirtz (the daughter of the notorious Dr. Heinz Doofenshmirtz). Unbeknownst to them, her brothers, Phineas Flynn and Ferb Fletcher, have already commenced their own intergalactic adventure to rescue their sister. Along with their friends Isabella Garcia-Shapiro, Baljeet Tjinder, and Buford van Stomm, the siblings hatch a plan to build a portal to the enigmatic planet Feebla-Oot. Their journey takes an astonishing turn as they encounter the quirks of Doofenshmirtz Evil Inc. Little does Candace realize that she is unwittingly caught up in her brothers' exploits.
As Vanessa manages to escape from a spaceship, Candace finds herself imprisoned on Feebla-Oot, where she meets the enigmatic leader, Super Super Big Doctor, who empathizes with her situation. Surprisingly, Candace learns that she isn't alone in her struggles; the benevolent Big Doctor also has two younger brothers who drive her to the brink. Big Doctor educates Candace about her special "Remarkalonium" properties, which the native Feeblans deem crucial, sparking a newfound sense of purpose within her.
However, when her brothers Phineas and Ferb attempt to mend their relationship with Candace by presenting her with a touching gift, their efforts fall flat, leading to their imprisonment alongside her—a fate comparable to that of Big Doctor's siblings. Meanwhile, Perry the Platypus covertly orchestrates their escape.
As Candace unveils the darker side of Feebla-Oot's "Remarkalonium"—realizing it is actually carbon dioxide necessary for the planet's mind-control spores—she inadvertently exposes Earth's natural exhaling processes. This prompts Big Doctor to devise a nefarious plan aimed at planetary domination. Candace and her crew unite with the oppressed Cowards, the star-crossed builders of Feebla-Oot, launching a courageous counteroffensive against the alien metropolis.
With Big Doctor and her minions fleeing to Earth, Candace comes to the sobering realization that her "Remarkalonium" is but a myth, and it is her bond with her brothers that truly defines her uniqueness. Upon returning to Earth to face the looming threat of Big Doctor, Phineas and Ferb manage to liberate Candace, who finds comfort in a coffee mug projecting nostalgic memories of their shared escapades.
As Mama unleashes its mind-controlling spores upon society, Candace seizes the moment to connect with Big Doctor, whose moment of clarity is cut short as Mama grows larger and devours her. The tension thickens among the siblings and their enigmatic caretaker, Mama, until an unexpected arrival shakes things up: Doofenshmirtz (accompanied by Vanessa and Perry) graces the scene with his unpredictable Chicken-Replace Inator. This outlandish device possesses the uncanny ability to swap anything or anyone with the nearest chicken, resulting in utter chaos.
Taking charge, Candace wields the device against Mama, which sends her miniature form tumbling back to the whimsical realm of Feebla-Oot. In an absurd twist, Mama's tiny stature cannot stand against the force of an alien elephant, which squashes her without mercy.
As pandemonium reigns, Big Doctor becomes an unwitting casualty amidst Mama's downfall, only to be captured by the Cowards—an enigmatic group thriving on mischief. Meanwhile, Candace is presented with a prime opportunity to apprehend her brothers for their repeated misdeeds, yet she chooses to let her mother, Linda, whisk her away, choosing instead to acknowledge her siblings' good intentions.
In a moment of intrigue, Perry receives a mysterious phone call from Major Monogram, congratulating him on a clandestine success—leaving everyone puzzled about the underlying implications. The atmosphere is charged with suspense as Perry's mission remains enshrouded in mystery.
In a surprising mid-credits scene, Lawrence stumbles upon the portal to Feebla-Oot, only to destroy it mere moments before Linda arrives, leaving audiences to speculate on the hidden secrets that this whimsical realm holds.
