It starts off at a backyard in the middle of Danville, where two inventive stepbrothers, Phineas Flynn and Ferb Fletcher along with their pet platypus, Perry, wondering how to make the most of their summer vacation every day.
"You know Ferb, we traveled through time and space, but we never traveled to a different universe... Ferb, I know what we're gonna do today!" Phineas said.
Then suddenly the boys' mother, Linda came in to the backyard and said. "Hi boys, I'm off to cooking class, and I'll be back late, and Candace is in charge."
"Woohoo!" yelled Candace, the boys' older sister from her room.
"Hey, where's Perry?" Phineas asked, it was a daily routine, everyday Perry disappeared to somewhere and then suddenly appear as if he appeared out of nowhere, and Phineas said, "Oh, there you are, Perry."
Meanwhile, Perry a.k.a. Agent P, entered his lair to receive his mission from Major Monogram.
"Ah, Agent P, it seems Doofenshmirtz has been trying to open a portal to other universes, or multiverse, he might try to recruit other villains to help him take over the Tri-State Area, so get over there and put a stop to it." Major Monogram instructed.
"Hey guys, what'cha doin'?" said a familiar voice.
It came from Isabella, with her were the Fireside Girls, Baljeet, Buford, and Irving.
"Inventing a portal that will takes us to different universes, or the multiverse." Phineas explained.
"It took a lot of study of the multiverse chain, and understanding how the portals work." Ferb explained.
Then suddenly Candace comes into the backyard and asks, "All right, what's going on here, and what's this?"
Phineas explains that it's a portal to different universes in the entire multiverse.
"You built a portal to other universes?" she asked, then said, "I'm in charge and that means you and your friends are not going."
"But you might have fun." said Isabella.
Candace thought for a moment, she did have nothing else to do then watch Phineas and Ferb.
"All right, fine, I'm convinced, let me come along." Candace said.
Candace decided to join the gang in their trip. But the question is where should they go. To be continued...