(Phineas, Ferb and Perry sit in a tree by their house in the backyard)
Phineas: Ferb, what do you wanna do today?
Ferb: (Shrugs)
Phineas: I don't know either. Do you know Perry?
Perry: Ckkkk. (Translation: I don't)
Phineas: Oh you don't know either? Now we don't know what to do today. Hmm…(thinks)
Linda: (walks from the house's door) Ok Candace, me and my husband will go see a movie. Take care of the boys , ok?
Candace: I gotcha, mom. (Walks in the house)
Phineas: And ummm… Wait! I got it! Ferb! I know what we're gonna do today!
(Later in the Sonic Underground universe)
Sonic: I just found this weird picture.
Sonia: What is it?
Sonic: A picture of a Dorito headed man and his bro with a square nose and square head.
Manic: Woah dude.
Sonic: IKR? Looks like someone drew this or it was a camera.
Manic: We think it was a camera which took a picture.
(The dimension remote from the Phineas And Ferb movie lands into Sonic's hand)
Sonic: What is that thing?
Sonia: It's a dimension remote. It travels you through dimensions.
Sonic: Cool! Can't wait to test this out! (Presses the remote's button)
(The button makes a green portal)
Sonic: Woah, it's a portal to a dimension!
(The 3 hedgehogs walk in the portal until they travel inside. The 3 hedgehogs scream as it happens)
(Later in the PAF universe, Phineas and Ferb work on their invention)
Phineas: Seems like we had just the nice touch to the stage. Well, we do need instruments.
(Candace walks out of the house's door)
Candace: (whistles) (Her eyes look at the stage) WHAT IS THAT?!
Phineas: The stage for the concert. How do you like it?
Candace: YOU GUYS DID THIS FOR THE CONCERT?! IMMA GO TELL MOM AND DAD ABOUT THIS. GRRR! (runs supersonic speed)
Isabella: (Walks to the backyard) Hello there Phineas! What you doin'?
Phineas: Making a concert!
Isabella: Nice. Hello there Ferb!
(Ferb waves his hand)
(The portal opens up)
Phineas: The dimension portal's back? What is this? Phineas And Ferb: Across The 2nd Dimension all over again?
(The 3 hedgehogs come out of the portal)
Phineas: Woah, who are you, where are you from and what species are you?
Ferb: Their names are Sonic, Sonia, Manic, they are from Planet Mobius and they are hedgehogs.
Phineas: That's…nice. They must have medallions because they're the band called Sonic Underground.
Sonic: That's us alright! (The 3 hedgehogs thumbs up)
Phineas: Now we need instruments.
Manic: Our medallions have instruments.
Phineas: BUT WHAT ABOUT INSTRUMENTS FOR ME AND FERB?!
Isabella: Just buy them at a store.
Phineas: Great idea! (Looks around) Hey, where's Perry?
(Cut to Perry dressing up with a hat on)
Background singers: Doobie doobie doo baa (x4)
(Perry goes into a garbage can. He lands into the place where he "Facetimes" Monogram)
Monogram: Hello there Agent P. I just wanted to let you know that Dr. Doofenshmirtz has teamed up with Dr. Robotnik, Sleet and Dingo to make the new SSSS Squad called: The Super Sonic Underground Search, Smash And Slam Squad Of Dr. Doofenshmirtz And Robotnik, at Doofenshmirtz's building. You will find a way to stop them from defeating the hedgehogs. Got it?
(Perry thumbs up, and gets into the ship)
(Cut to Phineas at an instrument store)
Worker: Aren't you too young for instruments like these?
Phineas: Yes, yes I am.
Worker: Cool. You can buy this guitar just for you…
Phineas: Thanks!
Worker:…And the bass for Ferb. By the way, you're welcome Phineas.
(Cut to the movie theater)
Candace: Mom! I just wanted to let you know that Phineas and Ferb are making a concert.
Linda: So what? How should I know?
Candace: You will drive me home when the movie's over to see this concert.
(Cut to Doofenshmirtz's building, with the evil jingle of Doofenshmirtz)
Doofenshmirtz: Robotnik, Sleet and Dingo, I present you this wonderful invention! Behold the slow-running-inator that will teach the hedgehogs a lesson!
Robotnik: Great plan for something like this!
Sleet: I agree with you!
Dingo: Me too!
Doofenshmirtz: With this inator it will stop the hedgehogs from running so fast!
Robotnik: Nice idea!
(Perry runs supersonic speed into the building)
All: Perry The Platypus?!
Perry: Hello there, Doofen-boi.
Doofenshmirtz: You can talk?! Well that's weird. Anyways, this slow-running-inator is converted into a remote, so yeah. Imma gonna give it a try.
(He presses the button and a laser that slows down the hedgehogs grows)
Doofenshmirtz: It's working! It's working!
(cut to the concert)
Phineas: Ok guys. This is all finished. Now we can do this concert after everyone comes in our backyard.
(cut to the Sonic Underground universe)
Knuckles: Hey, guys? Where are you? (picks up the dimension remote) Is this some remote or something? (presses the button)
(The green portal opens and Knuckles falls in and screams. He enters into the PAF universe)
Knuckles: Hey guys!
Sonia: Knuckles! I've seen you before!
Knuckles: That's right! I have the same look and personality!
Phineas: That's cool Knuckles! Can you be a lead singer?
Knuckles: At least I have the microphone medallion!
All: Cool!
(Everyone comes into the concert quickly while the 3 hedgehogs and Knuckles get their medallions out)
Phineas: (walks through curtain) Ok guys. This is a song about me and Ferb's life, featuring Sonic Underground And Knuckles!
(Everyone cheers. Curtain opens to reveal Phineas, Ferb, Sonic Underground And Knuckles)
(Song: Brothers Forever )
(In Acoustic)
Knuckles: (Singing) Phineas and Ferb were great inventors
They were one of a kind than a horn,
And they grew so much fame and they were popular than ever
But I'll tell you how they were born…
(Switches to Rock n Roll)
Phineas was born less than 15 years ago
And Ferb was born less than 15 too
They were born separate and had different last names
And when they were together, this is for you!
They were…
S, S, M, P, F: (Singing) Brothers forever!
Knuckles: (Singing) That's right!
They make great inventions for popularity and fame
They were…
S, S, M, P, F: (Singing) Brothers forever!
Knuckles: (Singing) Yes guys!
They were 2 families together and that's how they became…
All: (Singing) Brothers forever!
(guitar solo)
(Cut to the DE, inc. building, they talk during the solo)
Doofenshmirtz: OH NO! MY SLOW-RUNNING-INATOR HAS BEEN GONE BY THE SONG!
(Perry uses the biggest punching glove and he punches the villains)
Villains: CURSE YOU PERRY THE PLATYPUS!
(Back to stage)
S, S, M, P, F: (Singing) Brothers forever!
Knuckles: (Singing) That's right!
They make great inventions for popularity and fame
They were…
S, S, M, P, F: (Singing) Brothers forever!
Knuckles: (Singing) Yes guys!
They were 2 families together and that's how they became…
S, S, M, P, F: (Singing) Brothers, brothers, brothers, brothers forever!
Knuckles: (Singing with S, S, M, P, F) Yeah they were brothers forever!
(Everyone cheers)
(Sonic Underground And Knuckles put their medallions back)
(Everyone leaves the concert)
(Linda, Lawrence and Candace run to the fence to the backyard)
Candace: See? I told you!
Linda: Hey Phineas.
Phineas: Hey there, mom.
Lawrence: C'mon Candace. Let's go inside.
Candace: But i-(Linda snatches Candace)
Phineas: It was great performing with you all. I think we should do an invention some day.
The 3 hedgehogs and Knuckles: Yes we should!
(They all high-five at the same time)
Phineas: (Notices Perry) Oh, there you are Perry.
Sonic: Wow, a platypus. Cool.
(The end of the fanfiction)