Disclaimer: These characters are owned by Disney – but this plot is MINE! If you use it, I will hunt you down and … do something. I haven't worked out what yet.

This is my first P&F Fanfiction. I'm trying to make it was believable as possible. Once I can master a basic believable episode, I will be able to do more with it – so bear with me if this isn't the most exciting one you've ever read! Reviews, good and bad, would be appreciated, as if there's anything wrong/I need to work on I definitely want to improve it. Thank you, and sorry for rambling!

(Starts with a show on TV. The show is about an evil scientist battling an octopus. The octopus is in a cage wearing a belt)

Dr Snoozingworth (evil British scientist, resembles Doofenschmirtz): And now, Terry the Octopus, you will bow down to me superior intellectual abilities! (pulls cover off a giant laser cannon) I call it … the Opposite Day Ray! I, Dr Stein Snoozingworth, will use it to make everybody in the central-state-area the opposite of what they usually are! Nice people will become mean people! Mean people will become … you know, I'm not too happy about the whole "turn people nice" part but hey, you win some you lose some. Now, all I need to do is press this button, and – wha?!

Terry the Octopus: (leaps onto Snoozingworth and covers face with tentacles)

(Boys and Candace) (Phineas and Ferb are sitting on the couch watching, with Perry on Phineas' lap. Candace is standing next to them)

Candace: This cartoon is SO unbelievable!

Phineas: How do you mean, sis?

(TV) (Terry the Octopus blasts away in a hovercar as the laboratory explodes behind him)

Dr Snoozingworth: Curse you Terry the Octopus, and your ability to squeeze through incredibly narrow spaces!

Terry: (clacks beak)

(Boys & Candace)

Candace: Just look at that! (Terry flies the hovercraft away grinning) He's not even in water! Besides, how likely is it that some aquatic – even semi-aquatic – animal could pose as a mindless pet, then suddenly put on some clothing and become a secret agent fighting an evil scientist with an accent?!

(Boys look blankly at Candace)

Perry: (chatters)

Candace: AURGH!!! I don't know why I even waste my time …(storms away muttering)

Phineas: You know, Ferb, she does have a point about the whole fight-an-evil-genius part … but I likes the Opposite Day idea! I mean, not to that extreme, but imagine – it would be like the world was turned upside down! We could act differently, do things differently, see what it would be like if everything was the opposite … (snaps fingers) that's it! Ferb, I know what we're going to do today!

(Candace is now in her room. She is lying on her bed and talking on her cellphone)

Candace: (on phone to Stacy) … and then Jeremy is going to come over!

Stacy: Remind me again why that is?

Candace: Seriously, Stacy, you're as bad as my brothers. (Dreamily) Jeremy's coming over to help me look after Phineas and Ferb today, because Mom's band is playing at that Battle of the Bands Competition thing. She says if a helping hand will inspire me to not frantically call her in the middle of their performance, she's all for it – whatever that means.

Stacy: (a little weirded out) Oh-kay ... hey, I'll come over too – well, if you want, that is.

Candace: Sure, Stace, that'd be awesome. I'll see you there!

Stacy: See you later! (hangs up)

Linda: (walking past Candace's room) Candace, honey, the carpool's leaving now. I'll be back tonight – and remember what we talked about?

Candace: (exasperated) Yeees … I mustn't call you about the boys.

Linda: That's my girl. And don't worry – I don't think your brothers will be bothering you today. They're just practising handstands in the backyard. Bye!

Candace: Bye Mom! (to herself) "They're just practising handstands in the backyard" – not in this life! They're up to something, I just KNOW it!

(Phineas and Ferb are in the back garden. Ferb is typing on a laptop – Phineas is holding a clipboard. In the background, a crane, two men carrying a panel of glass, a giant floating baby head, a giant tower with a bell on top, two billboards advertising "Opposite Day – It's "Not" Fun!" and a bunch of balloons can be seen.)

Phineas: Right – we've built the bell tower, we've wired the giant screen, we're finishing off the programming of the command voice, we can walk on our hands … hey, we're ahead of schedule!

Isabella: (walks in) Hi, Phineas! Whatcha … (hears a blimp above and looks, seeing an advertisement for the opposite day) er … not doing?

Phineas: Only making the "worst" opposite day ever! But it hasn't started yet – we're going to have a grand opening in about an hour.
Isabella: Cool! So … what exactly do you do on opposite day?

Phineas: What do we do? What do we do?! Why, it's only the wackiest day of the year! Bullies become cowards, cowards become bullies – we say and do the opposite of what we mean! We can wear our clothes backwards, walk on our hands (thinks for a second) uh, make sandwiches with the bread on the inside … well, you get the idea. Anything goes!

Isabella: Wow – that sounds like fun!

Phineas: Well, that's the plan!

Isabella: So we say what we don't mean … hey, Phineas, can the Fireside Girls and I help out? We're trying to get our Reverse Psychology badges!

Phineas: Sure, everyone's invited!

Isabella: (grins) Great! See you later! (walks away)

Phineas: Hey, where's … oh, there it is (picks up his construction helmet).And Ferb, have you seen Perry?

Ferb: (shrugs)

Perry: (scuttles out of house, then making sure nobody is looking, puts on his hat opens a manhole, drops down into a speedboat and roars away. The speedboat roars through the sewers past a few alligators, then bounces out of the water, sending Perry flying. Perry lands in his chair)

Singers: PEERRRRYY!!

Major Monogram: Good morning, Agent P. Sorry about calling you in on such short notice, but not only have me got a mission for you, Doofenschmirtz is also up to something! You see, Agent P, since "Terry the Octopus", that brand new cartoon, has come out on TV, not only has Doofenschmirtz been buying huge quantities of dodgeballs over the Internet, but we think that the show may be raising suspicion about our organisation. We need you to see what Doofenschmirtz is up to – and see if you can find a way to stop this show once and for all.

Karl: Aw, man, but I love that show.

Major Monogram: Naturally, we can't please everybody. Now, go, go, GO!

Perry: (salutes and jumps into jet, blasting away)

Singers: Dooby dooba, dooby dooby dooba ... AGENT P!!!