P & F are sitting under the backyard tree. Someone speaks, and Phineas says, "Ferb, I know what we're going to do today...Hey, where's Perry? Cut To
Secret Agent HQ
Major Monogram tells Perry about Doofenshmirtz and how Perry has to stop him. Perry exits headquarters.
Scene cuts to whatever P&F are working on. Candace comes by and says she will bust them. Isabella offers the help of the Fireside Girls.
Perry enters lair and is immediately trapped. Doofenshmirtz tells him of his evil scheme.
The idea has been completed, and it's some weird device. At this point, chances are there will be a musical number with montage, or one will have happened when they were making the device. Candace will have called Mom, who thinks she's overreacting.
Dr. D has set up his device and is about to set it off when Perry attacks him, fights him, and ends up defeating him. The device will have gone off, however, but something will cause it to be directed towards Phineas & Ferb's device just as Mom gets to where the device is located. Perry will then turn off the "______inator."
Something will happen to their device, specifically whatever Doof's machine intended to do to the Tri-State Area. Candace will be jumping and pointing and telling Mom to look at it. By the time she gets to where the device was, Dr. D's machine will have taken care of it. Mom will look at Candace like her daughter is crazy and then invite the boys in for snacks.