Secret Agent Ferb the Platypus

When Phineas wonders what it would be like if he and his step-brother were platypi, Ferb creates a device that to turn people into them, and ends up turning himself into a platypus. He goes with Agent P on a mission, while Phineas and Candace try to find their missing brother and monotreme.

Episode Summary
It's another bright summer day in Danville. Phineas and Ferb wake up to Perry's chittering. Phineas states that he's so glad to have a pet platypus, and feels like going into song. He asks Ferb to start some music, and Ferb breaks out his guitar and begins playing "Everything's Better with Perry." Phineas stops him, as he feels that they've done that before. Instead, Phineas wonders what life would be like if he and Ferb were platypi, even if they wouldn't do much, they could still happiness to people's lives like Perry had done to them.

After Phineas leaves the room, Ferb begins to construct a machine to turn him and Phineas into platypi. While he is building it, he tests it out to see if it is working properly. A ray shoots out of it, and Ferb checks down in his checkbook that that is working well. The ray bounces off of a mirror on a dresser and hits Ferb. A blinding light fills the screen, and when it clears, Ferb is nowhere to be found. The camera looks down on the floor, and a platypus crawls out from under a pile of clothing. It is Ferb, who has been transformed, with no trace of his former self except for the green hair that has remained on his head.

He chirps in distress, and looks back at the machine. He activates it and shoots another ray at himself, hoping for it to turn back to normal. However, many rays shoot him, but he stays in his current form. Ferb crawls out of the room and, upon hearing Phineas coming down the hall, hides in a room, afraid to show his brother what he has become. Phineas calls out Ferb's name, wondering where he's gone, but Ferb can't answer. He decides to seek assistance from the only thing he knows nearby that shares his species - Perry.

Meanwhile, in the backyard, Perry dons his fedora and lifts a carpet that looks like grass from the ground, revealing the hole to his lair. He jumps inside and slides down a tube, landing in his chair. Major Monogram tells Agent P of his mission, that includes him, as usual, having to stop Dr. Doofenshmirtz. As well, Agent P will have a new recruit joining Agent P on his mission, to show the new recruit how things are done.

Ferb has reached the backyard, and is patrolling around it, looking for Perry. He steps on the grass carpet, and notices that it feels different from the rest of the grass. He lifts it, and finds the tube to Agent P's lair. Intrigued, Ferb goes down it. Major Monogram and Agent P salute as the screen turns off. Agent P walks towards his rocket car as Ferb slides down the tube and meets him. The two stare at each other for awhile. Agent P talks to Ferb in an Australian accent, as the viewers hear Perry and Ferb talking from a platypus point of view.

Agent P asks Ferb if he is the new recruit, but before Ferb can reply, Agent P asks where his fedora is, and hands him once. Ferb puts it on, and Agent P motions for him to come with him inside the rocket car. The two ride off. Meanwhile, Phineas explains to his parents that both Ferb and Perry are gone, and they find that kind of peculiar. Candace can find an upside to this, as since Ferb is gone, he and Phineas can't make some big contraption, and that she won't have to bust him. Phineas is appalled that Candace would think of busting at a time like this. Candace suggests that, since Phineas is so bothered by this, that they should ask the Mysterious Force.

Phineas questions what that is, and Candace explains that that is the thing that takes away his and Ferb's inventions everyday, and that it most likely took away Ferb and Perry. The boy lets that sink in, and, even though it's kind of silly, decides to do it anyway. He and Candace go to the backyard and call out to the Mysterious Force to bring the two back. After a silence, Phineas states that it isn't working, and that they should try another approach.

Ferb and Agent P jump in through the window of the Doofenshmirtz Evil Incorporated building. Ferb sees the doctor in a lab coat, and asks why they are in a pharmacist's apartment. Agent P explains that he is an evil scientist, and tells him to take off his fedora and act like a mindless pet and see what happens. The two do so, and Doof turns around and spots them, recognizing them as ordinary platypi. Perry puts on his hat and Ferb follows, to which then Doofenshmirtz sees that they are Perry the Platypus and another secret agent platypus he doesn't know. Ferb is surprised that Doof had called this platypus "Perry" to which Ferb realizes that Agent P is Perry, and that Perry is a secret agent. Ferb decides to act as if this has not fazed him, lest Agent P be suspicious of him.

Doofenshmirtz traps the two using rope that falls from the ceiling, and begins to tell his monologue. Agent P and Ferb chat, and Ferb states that he found Doof's ignorance quite funny. Agent P tells of the evil scientist's many backstory's which lead to the two laughing. Dr. Doofenshmirtz notices this, and tells them to listen up instead of goofing around. He talks about his Weed-inator, which he will use to plant weeds inside flower shops, therefore ruining its business. The two platypi chuckle about this as well, which makes Doof mad. He goes over to the two and yells at them. Agent P unties the rope, jumps up, and punches Doof in the face. Doofenshmirtz flies back as Ferb and Agent P charge at him.

Back at home, Phineas and friends are launching blimps into the air, with photos of Ferb and Perry on them, with the words "Have you seen these two?" flashing. Despite his friends' telling him to not worry, Phineas worries. Without his brother and pet, the redhead feels that much has gone out of his life. He sings Lonely, until a lawyer walks up to him and says that that song is copyrighted, and to sing it, Phineas would have to pay a fee. Phineas pays the lawyer, and begins to sing again, but the scene changes back to D.E.I.

Agent P and Ferb have Doofenshmirtz backed into a corner, but Doof activates the Weed-inator. It shoots a ray onto the streets, set for the Flower Shop. However, the ray bounces off the glass and back towards the building. Doof states that he should have taken into account the material of which the doors were made of, and he is then hit by the ray, and weeds sprout out from under his clothing, eventually covering him. He backs out of the window and begins to fall. Agent P and Ferb jump out and rescue him, with the two platypi deploying parachutes. As they soar above the town, Agent P and Ferb sing "A Brand New Best Friend (of the Same Species)", a duet about how, at first, they were just comrades out on the same mission, and then they progressed into best friends.

After the song ends, Agent P says that he must report back home, and that he will leave Ferb to put Doofenshmirtz somewhere. The agent leaves, and Ferb accidentally drops Doofenshmirtz on someone's lawn. A woman cutting bushes around her house spots him, and goes to cut what she believes to be a patch of weeds. Doofenshmirtz stands up and tells the woman that he is a person, but the woman does not believe him and thinks him to be a weed monster, and she chases down the street, slashing at him with her scissors.

Ferb glides to his house and drops from the parachute and into his room per an open window. He walks over to a toolbox on his bed, and, now used to his platypus body, is able to use his hands to finish the machine. He turns himself back into a human right as Phineas walks into the room, sadly sulking. He sees his brother and rushes to hug him. He asks Ferb where he was, to which Ferb replies that he was "around." Phineas then asks where Perry is, to which the monotreme then enters the room, and Phineas embraces him too. The boy leaves the room to go tell everyone else, leaving Ferb to pat Perry, finally knowing his secret, but not saying anything about it.

Meanwhile, Candace tries to get her parents attention to see the blimps that Phineas and friends made, but Linda & Lawrence are too busy trying to contact the Missing Child Association (they have been on hold for quite some time now). Doofenshmirtz is still being chased by the woman, and she hurls her scissors at him. Doof ducks, and the scissors fly through the air. It bounces off a clothesline and soars higher into the air. It pokes one of the blimps in the air, forming a small hole, and air leaves the blimp. It flies back, crashing into the others. The group of blimps go across the town, eventually coming near the Flynn-Fletcher house. They hit the house, causing it to topple over a bit. Candace, Linda, and Lawrence are tossed out of the house and onto the front lawn. The ball of blimps roll over the fence and onto the neighbor's property, and when Candace tells her parents to look at the sky, they find nothing.

Phineas races to Candace, his parents, and the kids in the backyard and tells them that Ferb and Perry are back. The gang goes inside to join the two in Phineas & Ferb's room for a group hug. Meanwhile, someone from the Missing Child Association has finally picked up the line and speaks, but finds that no one is there. The episode ends.