Phineas and Ferb plan to travel around the world on the longest day of the year; Linda and Lawrence leave for the weekend leaving Candace in charge. It's the summer solstice, so Phineas, Ferb, & their friends (along with the T.U.F.F.C.A.T.S.) decide to travel around the world in one day in an effort to create the "Biggest, Longest, Funnest summer day of all time". But not everyone believes they can actually achieve what they've set out to do, leading them to get some encouraging words, in the form of a duet, from Clay Aiken and Chaka Khan. Meanwhile, Dr. Doofenshmirtz takes a father-daughter trip to Tokyo with Vanessa.
Candace calls Jeremy in Paris to see how his trip is going, even though it's 2:00 a.m. from where he is. He tells Candace that he was dreaming of her, and they were in a Dixieland band with an iguana playing an Oboe. She soon hears some loud noises and sees Phineas and Ferb for their latest idea, which is a gigantic jump roping robot. Candace tries to call Linda, but she can't hear her daughter on account of the construction workers working on the street she's driving on. As usual, when she arrives, the robot is now gone.
Phineas and Ferb look through a scrapbook showing everything they've done so far over the summer, and with summer halfway over, Phineas suggests a real challenge, to make it the "Biggest, longest and funnest summer day of all time." Meanwhile, on a plane up above, Dr. Doofenshmirtz and his daughter, Vanessa, are on a plane to Tokyo. She wanted to go to Paris after dropping some hints, but Tokyo was more exotic, so she didn't mind. While they're flying, the pilot announces that there will be a delay. Doofenshmirtz is annoyed at how flights are constantly delayed and mentions 'turbulence, headwind, a giant-jump-roping robot...' and the jump roping robot that was with Phineas and Ferb is seen dangling from the plane.
Linda and Lawrence are leaving for the weekend, leaving Candace in charge. She tells Candace not to have any boys over, but as Jeremy's in Paris for a week, this won't be a problem. They also tell their daughter that being in charge means that she shouldn't be calling them whenever Phineas and Ferb are doing something bust-worthy. She needs to stop them on her own, because she's not doing her job, and she would also be in trouble for not stopping them in the first place. They leave, and Candace and the boys bid them farewell. The next day, at 5:00 in the morning, Candace dreams about Jeremy with French girls, and freaking out, she calls Stacy, who says she will come over. Candace also wants some breakfast, but gets hung up on instead. In the backyard, Isabella, her troop, and some other kids are outside wanting to know what they're doing up so early. Phineas tells them that they plan to make it the longest summer day ever. Isabella asks why did they build the Statue of Liberty, and Phineas tells her they didn't and removes the sheet. They intend to fly around the world, chasing the sun as they travel westward, making it the longest summer day ever. Buford doesn't believe they can do it, so he makes a bet. If they don't fly around the world "in one day," they have to stand in front of everyone, tell them they are governed by the laws of physics, summer isn't what they make of it, and more importantly, do absolutely nothing for the rest of the summer. If Phineas and Ferb do succeed, he'll eat a bug, but Phineas decides to add giving back the bikes he stole from everyone when they were little. With a duet from Clay Aiken and Chaka Khan, they sing I Believe We Can, and accidentally smash the plane's wing off. Perry gets a mission briefing from Carl, reading a note that Major Monogram hasn't been kidnapped and not to come looking for him. He tells Agent P that Doofenshmirtz is on vacation with his daughter to go to Japan where the Annual Good Guy Convention is being held and Carl assumes that it's a coincidence and gives him the day off. But Perry is immediately suspicious about this. Candace chats with Stacy about her relationship with Jeremy. Even though they dated, he's never called her his girlfriend and they've never kissed, which means he's single, in Paris, with French girls wanting his attention. Stacy tells her she needs to believe in herself. If she wants Jeremy to be her boyfriend, she has to call him her boyfriend first. Candace thought it was the other way around, so she takes the advice.
While picking up their luggage, Vanessa sees Major Monogram tied up and Dr. Doofenshmirtz introduces her to Major Monogram. She gets annoyed learning her dad had to bring his work with them when they were supposed to just spend some time together.
While the wing gets repaired, Phineas and Ferb have made out a map of their route, with stops in Tokyo and Paris to refuel. Isabella hears Paris, and Phineas invites her to come along. Buford is coming along with them to make sure they don't cheat and brings Baljeet with him. Candace tries to stop them from leaving until Stacy tells her to check out the map. When she sees one of the stops is in Paris, she decides to go with them. Stacy tells her this is a bad idea, since she promised her mom the boys would stay out of trouble. Candace tells Stacy that if she's with them, she can make sure they don't get into any trouble along the way and is aware that it's a massive rationalization. She says goodbye and tells Candace to see her cousins in Tokyo. The jet launches and takes off for Tokyo, leaving the Fireside Girls in charge of cleaning up the backyard and getting the party ready.
Vanessa and Doofenshmirtz are at the Tokyo Tower, and Vanessa is reading facts about it, however, it says nothing about a giant water balloon at the top. She immediately blames her father who reveals he plans to drop it on the entire International Good Guy Convention and pin the blame on Major Monogram to destroy his reputation. Therefore, it would put an end to the O.W.C.A.. In another part of Tokyo, Phineas and Ferb make a grindy and sparky landing to meet Stacy's family and her grandmother. Ferb gets out the plane and tells Stacy's grandmother who they are in Japanese, then she calls out the the other relatives who start to sing J-Pop (Welcome to Tokyo), during which they ask for some vegetable oil to refuel their plane and take off.
Vanessa is still mad at her dad for making their entire trip about his work and leaves for Paris. As she walks away, Perry appears and begins to untie Monogram while Doofenshmirtz tries to convince Vanessa to stay. He is about to push a button to make the balloon fall and to get the whole thing over with, but Perry stops him and causes him to drop the remote. The balloon swings around and knocks Vanessa off the Tokyo Tower. Luckily for her, the Sun-Beater 3000 flies past and she lands on it. She and Ferb recognize each other and the group lets her in. Phineas asks where she wants to go but she decides to stay with them, leaving Doofenshmirtz wondering why they're running off with her. He turns to Perry and Major Monogram, begging them to give him a ride to save her and the two reluctantly decide to help him The gang is flying over the Himalayas and Phineas tells them to keep an eye out for Klimpaloon, the magical old-timey bathing suit that lived there. Candace claims he's making it up and notices the altitude meter is beeping. Because Phineas and Ferb didn't calculate the weight of an extra body, the plane loses it's wings and crashes in Nepal, where Klimpaloon walks by. Buford says it looks like they're going to lose their bet, but Baljeet remembers that his Uncle Sabu lived near where they crashed and may be able to help them. Candace and Vanessa decide to stay and guard the plane while the kids head over to see Baljeet's uncle who isn't that far from them. When they arrive at his factory, he sings the song Rubber Bands, Rubber Balls and afterward Phineas asks to borrow one of the rubber band balls.
Candace and Vanessa go up the mountain for not getting a cellphone signal, whilst Doofenshmirtz is riding in Perry's hovercraft with Agent P and Major Monogram. Doofensmirtz is using a tracking device, which link up with the earrings Vanessa is wearing. When he spots his daughter on the mountain with Candace, they give him a parachute and an oxygen mask and fling him out of the plane because it was dangerous to land.
He fails to pull the cord and lands on the mountain, tumbling toward the two teens in a large snowball. They flee in panic and tell the boys (Who've finished making the giant rubber ball and are putting the plane on top of it) to start the plane. They try, but it keeps stalling and Phineas tells Ferb that he's flooding it, which Ferb denies. When they finally start it, Vanessa gets flung on board by the snowball but Candace narrowly avoids getting hit by it and it crashes into the plane instead, sending it off the cliff. Then it bounces back up, and she quickly gets on. Finally, the engine starts for real and they take off, blowing the snowball (And Dr. Doofenshmirtz) off of them.
Love Handel sings Bouncin' Around the World while they bounce through Singapore, India, China, Italy, Netherlands, Russia, Spain (accidently hitchhiking a Spanish seijuujin Heath Marella along the way), Egypt, Germany, Monaco, Australia and Belgium. They eventually run out of rubber bands and crash land in a river in Paris, France.
The Sun-Beater 3000 is washed up in Paris. Phineas tells everyone they need fuel and parts so they should all split up. Candace goes off to see Jeremy, Buford and Baljeet are to go get some more fuel, and Ferb goes to the Eiffel Tower with Vanessa to plot the directions back home. Phineas decides he will go look for some parts and invites Isabella to come along with him and she quickly agrees. On their trip, they came across the french/english speaking Seijuujin Wade Long and his Earth co-heart Kate Katoinette (whom Masami now had a romance with her, like Dave and Kitty) Candace walks around France in search of the hotel Jeremy is staying in. She comes across a French woman who knows both French and English, but Candace only assumes she only knows French. After failing to understand her, Candace leaves and spots Jeremy around the corner, she tries to say hi, but leaves dejected when she sees him with a bunch of French girls. Meanwhile, Isabella tries to seize a romantic moment with Phineas in Paris, and sings City of Love. While doing so, getting increasingly frustrated when he's too focused on trying to get the plane repaired.
Buford and Baljeet arrive at a cafe to get some vegetable oil for fuel. When Baljeet asks for some, the french man says no. Much to Baljeet's surprise, Buford steps in and reveals that he is fluent in French, he successfully gets the man to give them some oil. Phineas is also successful in finding some parts when he is given some large plastic baguettes that can be used to make their rocket float. On the Eiffel Tower, Vanessa chats with Ferb on about how her father seems to put his work ahead of her when she just wants to have a father-daughter vacation with him. Ferb tells her if you love someone, sometimes, you need to meet them halfway, seeing a flower shop, he leaves to get a flower for her while she thinks of what he said. Heinz suddenly shows up and Vanessa gets upset when she sees him with Monogram and Perry. She is mad at him for bringing work with him again, but he explains that all he did was hitch a ride with them and he had traveled halfway across the world for her. Realizing how much he did care for her, she goes with them. When Ferb returns, she's gone.
Everyone heads back to the plane which has the baguettes connected to it by rubber bands. When Phineas sees Ferb, he asks where Vanessa went, Ferb replies that she went off with someone else. Phineas thinks that's too bad since they were in the city of love so he thought romance would be the conclusion. This immediately angers Isabella (since she never spent a romantic time with him in Paris) so much she breaks her pencil, and Ferb gives her a new one. Then, Candace arrives and Phineas notices she doesn't look too happy. Candace explains that Jeremy had looked happy by himself and that she thought maybe she seemed creepy and obsessive for coming to visit him. Phineas says the same thing as he said to Ferb, making Isabella so angry her head explodes... or at least in her vision. Phineas then tells Candace to believe in herself like Clay Aiken and Chaka Khan said in their song, though Candace missed it, much to everyone's surprise. Candace is finding it hard to believe in anything, so Phineas reminds her of her past accomplishments from being a truck driver, to being the queen of Mars.
When Jeremy sees her on the bridge, he is surprised she's there so she tells him that her brothers are trying to go around the world in one day, but she accidentally blurts out that she saw him with French girls. He says that "I thought my girlfriend would have more trust in me than that", and upon hearing that, she calls him her boyfriend and tries to kiss him, but the jet is going away and she has to strap in and take off. He tells her he'll see her at the end of the week and they drift off. Candace sits next to Isabella and Isabella says that "At least you got a romantic love scene on a bridge." Isabella seem upset that almost everyone got a romantic love scene except her and Phineas. When they come to a giant frigate, the water balloon that had gone into orbit earlier falls into the ocean and creates a wave that allows them to hop over it. They jet through to the open ocean, realizing they may even make it to Danville ahead of time if nothing goes wrong. As soon as Phineas says that, the Sun-Beater 3000 begins to fall apart, leaving only the seats and them skipping across the ocean.
The gang now crash lands on a deserted island and they have one hour left until sundown. Candace comes back after exploring the island and is in hysterics when she returns, telling them that the only things on the island were them, a few trees, and a big ox. Buford laughs at his apparent victory and Candace gets upset that she'll never see Jeremy again. At first, Phineas decides to make a new plane out of materials on the island when he realizes he has nothing to build it with. Refusing to give up, he searches for every possibility in his mind, trying to think of ways to use the seats, one rubber band, and sand.
Dr. Doofenshmirtz and Vanessa thank Major Monogram and Perry for the ride and are about to leave, but Doofenshmirtz gets arrested by Perry and he and Major Monogram escort him away. Thinking fast, Vanessa holds the two up and tells them to release her father, and they let her escape with him in Perry's jet. Heinz thanks her, impressed with what she did, however, she tells him she improvised with a hair dryer. Doofenshmirtz says she's still a little evil and is so proud he begins to cry. Monogram decides to return back to HQ and Perry decides to stay in Paris for a bit.
At the deserted island, Phineas is desperately digging a very large hole in search of any useful minerals and only finds a sponge and a starfish. Isabella walks off on her own sadly and sits down to watch the sunset. As she begins to cry, Ferb approaches and offers her a handkerchief. She accepts and thanks him, telling him that she doesn't know what to do. They have been in Paris, yet Phineas hadn't even noticed her, she feels like giving up. Isabella says she would give anything if Phineas could sit with her and enjoy the beautiful sunset, but he was too busy still trying to find a way off the island. Phineas then runs up to her side and suggests digging a tunnel under the ocean. Realizing it wouldn't work, he gives up, sitting next to Isabella to enjoy the sunset. At first, she enjoys it, but after seeing how broken Phineas is that he'll lose his vision, she snaps him out of it and reminds him of the spectacular things he'd did that summer. She tries to tell Phineas that on Ferb's map, giving up was not part of their plan, but Phineas cuts her off and hugs her for reminding him that they still have Ferb's map, leaving Isabella in a trance-like state. Inspired, Phineas has everyone fold it into a giant paper airplane, which they put the seats to their original plane on. They use the last rubber band and place it around the two palm trees and use the ox to pull the rubber band back to make a slingshot. Candace is too reluctant, until Phineas reminds her it's the only way she'll ever see Jeremy again. The group is shot through the air, flying across the rest of the Atlantic Ocean, past New York, and heading towards the Tri-State Area. Then we see a meeting for 27th Substitute Teachers Day and the giant paper plane flies by and the speaker says "Alright! Who's the wise guy?" When the paper plane crashes lands five blocks away from where they live (Phineas remarks that it is their best landing of the day), they plan to run the rest of the way but stop when they see the construction site it had taken Linda an hour to go around. Buford is disappointed since he had realized what Phineas was trying to tell him and now it seemed hopeless. When he notices they are at his house, he gives everyone their bikes back, except Candace whom he had never stolen from so she is forced to ride a tricycle. At first, she did not listen to Phineas, making him yell at her for the first time because they only had 58 seconds according to Isabella. They quickly pedal down the road and launch off of a ramp and into their backyard where the party is awaiting, making it with no time to spare.
Linda and Lawrence arrive and see that they had a welcome home party, but they're exhausted and decide to let the kids enjoy it. They're especially proud of Candace for being so responsible, as they had no calls from her (which was mostly because Candace didn't have international coverage on her phone). Everyone sings and dances to the Summer Belongs to You, particularly Phineas, Isabella and Candace. Jeremy arrives from Paris early since he missed his girlfriend and sees the party. He and Candace both embrace and kiss each other. Isabella and Phineas hold hands and dance throughout much of the song. When the song ends, Phineas asks "Where's Perry?" and it is revealed that Perry is still in Paris, ordering at a French restaurant.
Guest Stars: Kate Katoinette, Heath Marella, and Wade Long joined along
Villians Guest Star: The Return of Karl and some-what brainwashed Sierra
Hero Guest Stars: The Loonatics Unleashed, Road Rovers, Phineas, Ferb, and their friends joined the T.U.F.F.C.A.T.S.
Monster of the week: The Eliminator