Theepisode opens with Jimmy attempting to find a friend to help him test his new invention. Since both Carl and Sheen are busy, he has no one to help him or to hang out with. He then sees Butch in the park with his little brother, having fun. Jimmy then begins to think that a little brother is exactly what he needs.
Later, Judy and Hugh of his thoughts using a side show presentation. He then requests a little brother. His mother is shocked and flustered, as is Hugh. They then try to explain that it is more difficult to give Jimmy a little brother than he thinks. Jimmy tries to debate that them having a son is simple, but Hugh interrupts him and offers his family pie.
Jimmy is then shown in his lab, saying that he will have to take matters into his own hands. First, the clock is ticking. Next, Jimmy fires an orange hat. Then, a screwdriver turns his shirt. Finally, Jimmy presses a red button, because his parents do not comply. He then proceeds to build a small robot in the darkness of his lab. He activates the robot by holding it up to the top of his lab until the thunder from the thunderstorm that went on outside at the time hit the robot. He then begins to talk about his new creation, saying he is twice as fun and energetic as any little brother before him. Then, as the robot turns on, he decides to name his new brother Brobot. Brobot then sits up and asks Jimmy what he wants to do, and Jimmy acts pleased with his work.
The scene then changes to Jimmy and Brobot in the park. Jimmy is bouncing on a Pogo stick that he altered so that it can jump higher, which is the invention that he had wanted his friends to test at the beginning of the episode. Meanwhile, Brobot is busy complimenting his new brother. Jimmy then offers the invention to Brobot, which his brother then uses, and he bounces up far higher than Jimmy did. Brobot returns to the ground, claiming that he jumped over the moon, and excitedly asks to go there. Jimmy responds that anything's possible.
Jimmy and Brobot are then shown playing baseball, with Jimmy doing terribly, but Brobot being impressed none-the-less. Carl and Sheen then walk up, greeting Jimmy and questioning him about Brobot. Jimmy then explains about how Brobot is now his brother and Carl and Sheen are impressed. Brobot then asks to attempt to hit the baseball and does much better than Jimmy had. Following that is a short montage of Jimmy and Brobot both attempting sports, with Brobot excelling past Jimmy in ability.
Sheen and Carl are then seen exclaiming how great Brobot is, making Jimmy upset. Jimmy then makes an effort to leave, with his two best friends only caring about if his brother can stay. Just then, Cindy and Libby show up, and are introduced to Brobot. The robot then approaches Cindy, whispering to her that Jimmy likes her. Jimmy protests while Brobot starts to tease him by singing "Jimmy's got a girlfriend". Libby, Cindy, Carl, and Sheen then leave with Brobot, leaving Jimmy feeling left out.
The scene then changes to Jimmy looking on as Hugh shows Brobot a photo album, and they seemingly ignore Jimmy. This continues as Judy brings snacks in for only Brobot and her husband, once again leaving Jimmy out.
Jimmy and Brobot are then seen in the lab, with Brobot continuing to insist that they play a game. Jimmy then begins to deactivate him which upsets Brobot, saying that Jimmy is his big brother, and they won't be able to play games anymore. Feeling guilty about this, Jimmy then sheds a tear which accidentally deactivates Brobot. Jimmy then says, "Oh, no! Brobot! Wh-what have I done?". But seconds later Brobot shows himself to still be functional and that he was tickled, excitedly saying that Jimmy made him indestructible.
Jimmy is then seen in his lab, looking on through his computer as Brobot spends time with his new parents. Goddard then opens his screen and replays the scene from earlier in the episode where Brobot told Cindy that Jimmy liked her. Jimmy then chases Goddard around his lab, telling him to stop, and the episode ends.
Phineas and Ferb build robot versions of themselves, Phinedroids and Ferbots, to help out with their Big Ideas. Meanwhile, Dr. Doofenshmirtz tries to remove all the messages he left on his girlfriend's answering machine.
The boys are sitting under the tree in the back yard, going through the candidates for what awesome thing they will do today. Among the choices are a completely food-based water park, a tunnel to the center of the Earth, a chamber of invisibility, and a microscopic zoo that rests on the head of a pin. As the realization sinks in that they have a lot of blueprints, models and schematics, a falling leaf prompts Phineas to comment that "summer doesn't last forever and we've got too many plans for just the two of us. We either need more days of summer, or more of us." Ferb hands over his plans for a robot, and that gives Phineas an idea: make Phinedroids and Ferbots.
At this moment, Phineas notices Perry is missing. Perry is up on the roof of their house, pressing bricks on the chimney. The chimney opens up to reveal an elevator. During the trip down to his lair, it makes a stop and Santa Claus gets in. Perry gets off, revealing a sign that says "Chimmney Vator". Major Monogram begins the briefing, telling Agent P that Dr. Doofenshmirtz has made 57 phone calls in the past hour, but he doesn't know to whom or why. Agent P points back towards the elevator door, but Major Monogram says it's not to Santa. As Agent P leaves, Major Monogram tells Carl, who is off-screen, that this is why he doesn't like using the Chimmney Vator, beside the fact that Santa owed him a pony.
Back in the garage, the boys are finishing up the robot-building device. Phineas places pictures of them on the scanner. Within moments, the robots are done in the new version of the device. The one sitting in the background is deemed an "old dinosaur". As pairs of the Phinedroids and Ferbots march out of the new device, Phineas starts to welcome them. He notices that Ferb's bullhorn is not on, even though Ferb isn't saying anything. He continues and asks the robots to each take a blueprint and report to the back yard. Phineas asks Ferb to do the honors, which is to select "dance mode" on the remote control. The robots begin dancing and sing Phinedroids and Ferbots. The boys join in and soon, everyone is "busting chillin' moves with their robot poses".
Meanwhile, at Doofenshmirtz Evil Incorporated, Dr. Doofenshmirtz is scrunched up in a red chair, muttering "why won't she pick up?" Agent P lets himself in the room using a key that Dr. Doofenshmirtz has given him. Dr. Doofenshmirtz thanks him. "Much more civilized than crashing through my ceiling, don't you think? Yes." Dr. Doofenshmirtz then tries to persuade Agent P that he isn't up to anything, just dealing with a "relationship bump" with his new girlfriend. Agent P glances over to the computer, where there's a plan for a crane-mounted magnet. It will help him erase the messages he's left on her answering machine. Agent P gives him a disapproving look, so Dr. Doofenshmirtz activates a rope trap. "Too bad for you. Too bad you don't believe in love", he says. As he drives off in the crane, he yells, "Too bad!"
Meanwhile back at the house, Candace is on the phone with Stacy. She can't watch Stacy get her ears pierced because she is in charge while Linda Flynn-Fletcher is over at Zippy Pics. A Phinedroid walks by the window, carrying a ladder. At first, Candace doesn't think this is unusual, until a second Phinedroid walks by, carrying the back end of the same ladder. Candace ends her call and goes to the backyard and is confronted by the sight of multiple Phineases and Ferbs. She concludes that her brothers are robots. "They're brobots!" She freaks out, runs through the house and climbs down the ladder into her Panic Room. Clutching Mr. Miggins, she calls her mom. "Phineas and Ferb, and and Phineas and Ferb, and and robots. Chamber of invisibility. Elephants." she squeaks.
Mom's heard this all before and tells Candace she'll see the robots after she gets home. Candace knows everything will be gone by that time. But as she talks to Mr. Miggins, she realizes that if she captures one she'll have proof. After donning protective gear, she sets out a can of oil as bait. A Ferbot takes a drink, and Candace catches it in a burlap bag. Candace dumps the bag in front of the house, then calls Stacy to let her know what she's done. She doesn't get very far because Stacy is crying. A few moments later, Stacy's mom comes on the phone so Candace pretends she didn't know anything about the ear piercing. Behind her, a garbage truck has picked up the burlap bag, followed by a Fred's Fertilizer truck dropping off a bag of fertilizer. Candace is thinking about "poor, mixed-up Stacy" and doesn't notice the change.
Over in the backyard, Phineas and Ferb check on the robots' progress. The Chamber of Invisibility is completed. Phineas congratulates the Ferbot, then notes he'd normally tell Ferb that. Next is the microscopic zoo. Phineas climbs up the ladder and peers through the eyepiece of the giant microscope. That's a success, too.
As Phineas looks at the tunnel to the center of the Earth, he begins to realize that it isn't as fun when others do their projects for them. At this moment a Phinedroid delivers a new plan to him. It's for the hovercraft lounge chair, which the Phinedroid has already completed and rides away on. This was the plan that the boys were going to do the following week. Phineas sees it's getting worse. The robots have run out of projects and are designing their own. Phineas asks, "Have we learned nothing from Mary Shelley's Frankenstein?" Then he notices the coffee stand. He pronounces the overpriced coffee stand as "so Nineties".
Just then, the coffee creates a new problem: the robots start going haywire. One can't sleep. When one falls down in front of him, Phineas decides to shut down the coffee stand. The Phinedroids and Ferbots don't like it. Phineas tries to execute the shutdown command but it doesn't work. The robots start closing in on the boys.
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