This story takes place in the times when Phineas, Ferb, Isabella, everyone turns to 16. Candace and Heather are now 20, and became rivals with each other among Jeremy. New guys from the new guys stage, Henry, Ralph, Claire, Margaret, Susan and Al come to Danville, and every new character except for Perfect Peter are all 16. Peter is now 11.
In the returning summer vacation. Phineas and Ferb were still up to devising something even when they have turned into teenagers. Isabella was being happy about liking Phineas, Candace already went to college. Heather, sister of Hamilton and Howard. Phineas and Isabella's 'first love' still lasted, even though Hamilton was preparing for a crackdown for this lovely balance (This doesn't mean that he is a bad guy. He is still one of the best friends of Phineas and Ferb.).
The 1st day of vacation began. Isabella met Emily and started shopping near Danville downtown, from the morning. However, Phineas and Ferb were still at their home thinking what to do that day. When they turned the radio on, they heard an advertisement: 'You choosing a dating place for your girlfriend or a place for fun? Then come to the Danville Downtown street party at the street of Fair!' Then, Phineas shouted, "Aha! Ferb, I know what we are gonna do today!"
However, Phineas never knew that there was Agent E, which was Hamilton's eagle on their tree. Agent E had a secret eagle code of communicating with Hamilton and Howard's SIOA America headquarter (The Grand Headquarter was in The Hague, United Netherlands) in Danville downtown. Of course, Hamilton hid the SIOA headquarter into a skyscraper, only 23 meters than the Doofenshmirtz Evil Incorporated, which was on the other side of the Danville skyline. Hamilton was doing his job there as the Headquarter leader, or the Executive Officer of the SIOA. Hamilton and Howard were in the SIOA America headquarter, just at the 50th floor, which did the communication with the secret agents all over the nation. When Howard saw the alert, He called Hamilton about the message. When Hamilton saw the camera in the agent's eye and the signal of the agent, he was really shocked. When he also saw the missed message-the message of Phineas and Isabella's first date from Agent Peter. When he saw the part when they had their first kiss in the ferris wheel by the camera, he could have just been knocked out. Finally, Hamilton called the Superior Executive Army Leading Strikers(or the SEALS) at the basement army floor, which is almost forbidden. He shouted, "Soldiers! We have got a mission. We are going to stop the date of Phineas and Isabella!" Suddenly, Officer Hadros asked, "Wait, Executive Troop Master Hamilton, you're starting this battle because you like Isabella, isn't it?" Then, Hamilton answered, "What? I like Isabella? Well, sort of... Anyway, love does not count on military, especially here, in the SEALS! Anyway, troops, get ready for battle!"
However, Phineas called Isabella on the phone, "Hey, Isabella!" Then, Isabella answered, "Huh? Phineas?" Then, Phineas said, "Well, Isabella, you are in downtown, right?" Then, Isabella answered, 'Well, yes... So?" Then, Phineas said what he wanted, 'Well, Isabella, would you go to the Danville Downtown Street Party with me?' This meant a date with Isabella. Then, Isabella said, "Okay, I'll be waiting at the Danville Statue. Please pickup me!" But, Isabella was really screaming by her head: 'AHHHHHHHHHHHHHHHH!' After the phone call, Phineas got his own orange convertible that he used to drive.
At Phineas' house, Phineas was preparing his costume for the date again. When he was ready, he asked Ferb, "Well... Ferb! How do I look? Don't I look awesome?" Then, Ferb answered, "Well, pretty good?" Then, Candace came to their room (Candace was in Vacation) and said, "Huh? What are they up to? Phineas in love with Isabella? What reason would she like a bad guy like you?" But, Phineas ignored. Then, Phineas was just finding the key for his orange convertible.
However, when Hamilton heard that from the headquarter, he was exploding. Finally, he said, "All right, soldiers! This is a very important mission, so I'll go directly to the mission. Any complaints?" Then, the basement was silent. Everyone agreed on his plan for the direct participation of Hamilton. Then, Howard said, "Now, soldiers! Let's get prepared for the mission! We gotta move fast to the Street of Fair if we don't want to spoil our mission!" So the soldiers have been preparing the mission for stopping Phineas and Isabella's date.
However, in the Doofenshmirtz Evil Incorporated, Perry the Platypus was still fighting Doofenshmirtz. This time, Doofenshmirtz made the break-up-inator. The break-up-inator can make one's brain into nothing, so he could attack anyone. Also, he has inprisoned Perry the Platypus in a Platypus cage specially for arresting Perry the Platypus. Then, Vanessa came in and tried to bust her father by telling this thing to her mother.
However, Candace and Heather was having a fight over Jeremy. Heather shouted, "Hey! Candy-like Candace! Stop resisting! Jeremy's mind has changed! He no longer likes you! He's mine!" However, Candace yelled, "Of course not, you feathery Heather! Jeremy still likes me!" The fight continued this way. Finally, Jeremy came. When Jeremy saw the fight, he couldn't choose who he really liked. Well, before Heather moved in, he liked almost only Candace, but when Heather came, his mind has been balanced between Candace and Heather. Anyway, he was really shocked when he saw the fight.
Back to Phineas and Isabella. As usual, Phineas drove the orange convertible to where Isabella was. Then he said, "Oh, there you were, Isabella. Whatchu doin'?" Then, Isabella said, "Um... Just waiting for you...?" Then, they went to the Street of Fair, where was a bit far away from the statue. There, the plan of Hamilton began. At first, he was only going to make something change in their dates-stop any stuff like kissing each other during the date. However, the Wireless Communication Device was suddenly broken, so it was impossible to communicate with the teams on operation. The team's situation was worse. There was no conduction from the headquarter. According to the Army Emergency Response Law (or the AERL) of the SIOA, when communications don't work, all agents in mission are on Directing Colonel-conduct Battle Mode (or DCBM). And even worse, half of the trucks' engine didn't work, and few of them ran out of fuel. It was a big problem for the agents. So, they followed one of the AERL again-if the problem gets bigger during DCBM, then transfer into EDCBM, which emergency situation is included.
However, Hamilton succeed one part of his plan-the fair is closed. However, Phineas suggested, "Eh? The fair should be open now! But... Anyway, let's go to somewhere else... so, where do you want to go now?" Then, Isabella said, "Well... just anywhere....?" Phineas considered for a long time. Then, he hardly opened his mouth, "Um... There is a place to go near Danville... I'll take you there if you want to...?" Then, Isabella said, "Well, anywhere's okay..." The place that Phineas was talking about was a restaurant. Phineas and Isabella had a perfect time. They had talks of romance... However, there was agent Ferb, who is now an agent of the SIOA. Ferb is not on Phineas' side anymore, because because of his love with Isabella, Ferb couldn't do his work with Phineas anymore. In fact, he betrayed him. He was a traitor.
After that report from Ferb, Hamilton could get good data about what Phineas and Isabella were doing. Just at time, the wireless communication system worked again, so Hamilton could contact the soldiers again. Back to Phineas and Isabella. After the meal, Phineas and Isabella came out. When the couple was about to get in the convertible, Isabella said, "Well, that's a good meal... Anyway, I'll be right back. Just wait a second." There, Phineas found something really new-the fact that something wasn't going right. There, he saw some people near the car with black uniforms with a skull on it. Phineas thought, 'Wait a second, this is not Hamilton's army... His uniform makes me feel much different...'