The episode begins on a summer day as always...ten years later. Phineas and Ferb are now teenagers set to go to college. Candace is at law school, Baljeet has finished college and become a professor, and Buford has decided to attend film school. Ferb is set to attend college in the UK, while Phineas narrows his choices down to Danville U, or Tri-State State.
Meanwhile, in the intervening years, becoming a high school teacher has caused Doofenshmirtz to give up his evil ways (following from the previous episode, Doof 101), thus leading Major Monogram and Perry to retire from active duty. Carl in turn became a commander of O.W.C.A., with Irving as his intern. When the -inator alarm is reactivated, Perry is called out of retirement to find out what Doof is up to.
Back at the Flynn-Fletcher residence, Baljeet mentions off-handedly that Isabella will be leaving Danville and attending Tri-State State. Upon Phineas mentioning that Isabella hasn't come to visit during the summer, Baljeet questioned why Phineas and Isabella weren't together. He and Buford mention Isabella's long-time crush on Phineas, who is shocked and was always under the impression he's in the friend zone. Ferb also knowing about the crush causes Phineas to freak out and step out to clear his head. Nobody told him because Baljeet says men don't share each other's feelings.
When Perry confronts Doof about setting off the old -inator alarm, Doof mentions that the -inator he created isn't for any nefarious scheme; it's a basic gold chain necklace that's meant to give Doof a Mid-Life Crisis so he can fit in with other people his age. Upon wearing it, Doof drags Perry along for the rest of the episode, partaking in more and more dangerous stunts, from jet-skiing, to reckless driving, to skydiving. Eventually, Doof tires of the Mid-Life Crisis-inator and throws it away, only to accidentally land on an unsuspecting Lawrence.
Isabella and the Fireside Girls are also teenagers now, with the former working as a waitress in her mother's restaurant. Like Buford, Baljeet and Ferb, the Fireside Girls also know of Isabella's crush on Phineas, and decide to work with the three boys to hook the two of them up before Isabella leaves. Phineas, meanwhile meets Isabella's Nana, who tells him that Isabella is leaving in the afternoon because she starts college early. The three guys and the Fireside Girls's romantic setup in the backyard is completed, but is inadvertently destroyed by a runaway dumpster caused by one of Doof's stunts.
With Isabella's shift at the restaurant done, she says goodbye to her mom and drives away. A chance glance at a photo of Phineas in her wallet causes her to reminisce about all the summers they spent together, and laments at what life might have been like if she and Phineas managed to get together. Similar thoughts roll in the mind of Phineas when he reaches the restaurant too late to see her off. He realizes that, in hindsight, all of Isabella's advances had been extremely obvious and he never took a hint.
Phineas, downcast, eventually reaches Isabella's house, only to see her gone; however, a truck moves away, revealing her on the steps of his house. As the two sit together, Isabella says that she couldn't leave without saying goodbye to Phineas, and they talk. Phineas tells her about the crush, and Isabella admits that she did have a "big crush" on him during their youth, but gave up on it during high school. Phineas, in turn, admits that it is precisely during that time when he started having feelings for her. The two are saddened by the bad timing, but when Isabella mentions she's going to Tri-State State, Phineas makes up his mind and says that he'll be going too. With applause from Baljeet, Buford, Ferb, and the Fireside Girls, Phineas and Isabella hug and begin their relationship, promising to see each other in two weeks.
After Isabella drives off, Phineas has one last-minute decision. Thanks to Ferb and Vanessa (who are a couple by this point), Phineas managed to intercept Isabella at a traffic light. Stepping out of the car, the two finally kiss. After Vanessa and Ferb drive off and joke at how obvious the relationship was, Isabella remarks that, in the end, it was worth the wait. Phineas agrees, and the episode ends with the two, officially a couple, kissing again.
- Accidental Misnaming: Irving says that the -inator alert is coming from a "Dr. Dooferdsmits."
- Affectionate Nickname: Vanessa calls Ferb "Ferbs" as she meets him at the Flynn-Fletcher house.
- Be Yourself: At the end, Doofenshmirtz decides to ditch the Mid-Life Crisis-inator and go back to his slow-paced lifestyle and confirms his bowling plans with Perry.
- The Big Damn Kiss: Phineas and Isabella finally get theirs.
- Book-Ends: The episode begins and ends with Dan and Swampy in live action, sitting next to a pile of viewer mail.
- Borrowed Catchphrase: Phineas greets Isabella with her own catchphrase.
- Likewise, Commander Carl uses his old boss's "Great googly moogly!"
- Brick Joke: During his mid-life crisis, Doof purchases and crashes a sports car, but the car dealer offers him another one, this time in yellow. At the end, Vanessa shows up driving said car.
- Call-Back: Buford describes the romantic atmosphere as, "it will be just like love kicked them both in the face." Later, when Buford shows up at the end, after all is said and done, Baljeet describes what happened as "it was just as if love kicked them both in the face."
- The Cameo: Candace only appears briefly as a law school student.
- Chekhov's Gun: At the beginning, Phineas decides to naturally pick two viable invitations for college. When Isabelle reveals that her choice happened to be the same one Phineas got an invitation for, meaning that they can still be together.
- Continuity Nod: Several, as is the nature of the show;
- Baljeet mentions how Isabella clearly had hearts in her eyes as was shown all the way back in the first episode. And they apparently weren't a visual effect; she somehow physically changed her eyeballs into hearts.
- During "What Might Have Been," several scenes of Isabella showing her affection are shown when Phineas is lamenting on how, in hindsight, quite obvious she was.
- Buford again dresses like Cupid, as he did in "That Sinking Feeling," with Baljeet even mentioning the events of that episode.
- As pointed out on this episode's page on the show's official wiki, Phineas answering a question with "this" and then giving The Big Damn Kiss, is similar to what Jeremy did in "Summer Belongs to You"
- Continuity Snarl: Carl mentions that Doof gave up evil after being placed as a teacher in Vanessa' high school, as seen in "Doof 101," but in the actual finale he gave up being evil after Vanessa pointed out to him that he wasn't really a bad guy at heart.
- Contrived Coincidence:
- A dumpster hooked by Doof's sports car manages to take away the romantic dinner set up by the teens. Lampshaded by Katie "wow, now it's exactly like it was when we were kids!"
- Isabelle solemnly reveals to Phineas which college she chose to attend, which happened to be the same one Phineas got an invite to.
- Department of Redundancy Department: Gretchen mentions how they have special romantic songs and balloons in Phineas and Isabella's favorite colors to help them "think about love." When Katie asks what they'll do if neither of those work, Buford reveals he has a sign reading "think about love."
- Distant Duet: "What Might Have Been."
- Distant Finale: The true Grand Finale, set ten years in the future.
- Everyone Can See It: By the time of this episode, everyone is completely aware of the UST going on between Phineas and Isabella. Heck, even Vanessa Doofenshmirtz and Isabella's mother knew about Isabella's crush on Phineas! This leads their friends to do everything they can to get them together.
- Foregone Conclusion: If Quantum Boogaloo is any indication, revealing that by the time the boys are 30, Phineas and Isabella stayed together. Then there's Doofenshmirtz's HeelFace Turn, which happens in the actual finale.
- Gambit Pileup: Baljeet and Ginger constantly try to manipulate and outwit each other. Baljeet is aware of this and it apparently keeps them in a healthy relationship.
- Green-Eyed Monster: Ginger is not pleased when Baljeet brings up his former crush Mishti.
- Hand Wave: Why didn't anyone tell Phineas about how Isabella felt about him all those years? Because they're guys, they don't talk about their feelings.
- Happily Ever After: Being set ten years after the real Series Finale, Phineas and Isabella finally get theirs.
- Hollywood Midlife Crisis: Played. Dr Doofenshmirtz hasn't had a mid-life crisis and feels left out compared to other men, so he invents a device that induces a mid-life crisis in the wearer!
- I Need to Go Iron My Dog: Dan leaves at the end, apparently embarrassed by Swampy, who was touched by the emotional ending, by saying he's "got a thing."
- Mood Whiplash: A Played for Laughs version; Phineas handing Isabella the college acceptance letter is played as a big romantic gesture, but after he does, the background music stops and she is disappointed, until he realizes he gave her the wrong letter.
- Oblivious to Love: On both ends. Phineas never figured out Isabella had a massive crush on him for the last ten years until he was flat-out told by his friends, while Isabella remained completely blind to Phineas's affections throughout high school (which were greatly implied to be as obvious as her affections were for him).
- Phineas' verses of "What Might Have Been" actually point this out with him singing that, looking back, he realizes that she was being quite obvious in her flirtations.
- Official Couple: After four seasons of teases, a memory wipe, and hints in the Star Wars special, note Phineas and Isabella are finally together. After the Time Skip, Baljeet and Ginger are also together, as well as Ferb and Vanessa.
- The Reveal: Albeit a retroactive one via Word of God, but this episode finally revealed that Ferb is short for...Ferbs.
- Sealed with a Kiss: The episode proper ends with Phineas and Isabella kissing after finally affirming their relationship.
- Series Fauxnale: Was speculated to be this by many fans, due to its epilogue-like nature and finally dealing with the show's longest lasting character relationship. However, there were two episodes left to air, including the narrative finale "Last Day of Summer."
- Shout-Out: "Phinabella" the shipping name of Phineas and Isabella among the fandom, is utilized in the song "What Might Have Been".
- Spell My Name with an "S": Milly's name is misspelled as "Millie" in the credits.
- Teens Are Short: Averted, Isabella is as tall as her mom, and the other kids are pretty tall as well.
- Time Skip: Takes place ten years after the series.
- Unrequited Love Switcheroo: Isabella finally gave up on Phineas around high school, thinking it would never happen. That was when Phineas started developing feelings for her, but he never told her because he was fully convinced that he was stuck in the friend-zone.
- What Happened to the Mouse?: The fates of several supporting characters (Jeremy, Jenny, Suzy, Charlene, Mayor Roger, and Norm, to name a few) post-Time Skip go unspoken of here.
- Worth It: Isabella's reaction to finally getting her kiss from Phineas is that it was worth the wait.