Linda tells Phineas, Ferb, Isabella, Baljeet, and Buford that she will be going over to Vivian's house for a while. Buford asks what sort of fun they could have while it's raining, and the gang seems stumped. Irving suggests Hide and Seek, which surprises everyone. Irving then proceeds to say that he is their biggest fan and shows off his photo album of their Big Ideas. Phineas agrees that this is a good idea, but Buford states that the house is too small for a good game of hiding and seek. Ferb suggests that instead of the house being too small, they are too big.
This gives Phineas an idea and says, "I know what we're going to do today!" This excites Irving and then asks if he can say, "Where's Perry?" This confuses Phineas and later frightens him a little as Irving tries to say the line in dramatic fashions. Meanwhile, in Perry's underground hideout, Major Monogram informs Perry about a series of disturbing messages Doofenshmirtz has left on his answering machine, asking for Perry to come over to stop him from being evil. Monogram tells Perry to stop him while other messages continue to play in the background.
Phineas and Ferb finish their shrinking machine and has everyone touch the red ball on top of it. Everyone starts examining the world from their new size. Meanwhile, Perry barges in on Doofenshmirtz, whose evil deed for the day is throwing recyclables out in the regular garbage. He says he has seen the evil of his ways and starts to pat Perry on his back to shoo him home. In doing so, he plants a minibot on Perry's back, without him knowing about it. Phineas declares Candace as it, and everyone hides. Candace, meanwhile, is talking to Stacy on the phone and doesn't notice the gang.
She does, however, spot the shrinking machine and rushes to take it to Linda across the street. She picks up an umbrella, which Baljeet is hiding on, and flings him up onto the light fixture. Phineas tries to think of a way to get the shrinker back, and Irving is disappointed when he learns that his heroes are just improvising the whole time. Baljeet feels the chill of the air conditioner blowing down on him. Perry returns home and goes to sleep on his pet bed. The minibot comes alive and Dr. Doofenshmirtz steers it in the hopes of finding Perry's address somewhere. As Candace crosses the street, it stops raining. She knocks on the Garcia-Shapiro front door, but accidentally activates the shrinking machine on herself. Vivian answers the door immediately afterward, seeing only the device and takes it inside. Candace then faces the horrors of the front lawn.
The minibot spots the children, but Doofenshmirtz thinks the signals got crossed since they look regular sized. He loses control of the robot, and it chases after the children. They use a sewing basket to tie threads over it and pull it down onto the ground. Ferb reprograms the remains of the bot to be a mini-fly, and Irving takes the opportunity to get a photo of himself with the gang. They go to rescue Baljeet, who survived by hiding in a dead fly's corpse. Candace is saved from Pinky by Isabella and helped onboard the mechanical fly. They soar over to the machine and return to their normal sizes. Candace then runs off to show Linda the inventions. Doofenshmirtz regains video feed, and Perry spies the camera watching him. He crushes it, which prompts Doofenshmirtz to activate the self-destruct button on his bot. It destroys the shrinker with it, somehow leaving the table and the rest of the kitchen unharmed. Candace swears it was here upon not seeing it, but Linda and Vivian don't believe her, thinking its teenage hormones. Doofenshmirtz comments on feeling empty that he has no come-uppance after destroying his robot.