Phineas and Ferb woke up in the morning. Phineas reminded Ferb that it's Phineas' birthday, which makes him excited. Ferb points to the calendar and Phineas realizes it's Isabella's Birthday as well. Phineas decides to throw a huge party for him and Isabella, mostly Isabella. Phineas notices the absence of Perry. Perry opens up the TV and jumps into a slide. Major Monogram tells Perry that Doofenshmirtz has been making a Antibirthdayinator. Perry remembers the birthday and dashes out. Phineas is checking off his shipment of balloons, snappers, tables, chairs, and tablecloths. The shipping guy asks Phineas if he is to young to use all this stuff, and Phineas is surprised and says "No, no I'm not". Phineas realizes he has to keep Isabella away while they are working. Phineas informs the Fireside girls, who do their best to stall her. Perry breaks into Doofenshmirtz's building and is trapped in chains. Doofenshmirtz reminds him how his birthday never worked out, so he will ruin it for others. Doofenshmirtz shows him the Antibirthdayinator, which looks a lot like the rainbowinator. The Fireside girls swarm Isabella and keep her at her house. Phineas is just about done, and tells the Fireside girls they can stop. They did, and Isabella walks over to Phineas' backyard. She asks her catch phrase, and Phineas shows her the party. Isabella is thrilled at the sight, and thanks Phineas half a dozen times. Doofenshmirtz looks around for a plug, and finds that Agent P is sitting over it. Agent P escapes his chains because the chains were too big, but is too late as Doofenshmirtz plugs it in and aims right at Isabella's and Phineas' party. Perry then trips over his chains and Doofenshmirtz fires. Isabella and Phineas are having a good time, until the beam gets rid of the party. Isabella is upset, but Ferb reassures her that she should tell Phineas her secret as a gift, then this will be a good party. Isabella sucks up the courage and tells Phineas, and he says he felt the same about her all the time, and they kiss, to which everyone aws. Linda and Candace come, and both are happy at the sight they see. Isabella remarks this as the best birthday she's had, and Phineas couldn't agree more. Perry hits the reverse button on the Antibirthdayinator, and shoots a beam at Phineas' backyard. The party and the decorations come back, and the party can continue, but Isabella and Phineas have already given their gifts to each other; love. Perry comes back with his usual chatter.
The "too young" line
Shipping guy: Aren't you a little too young to buy this stuff?
Phineas: A bunch of party decorations? No, no I'm not.
Isabella: Now how are we supposed to have a happy birthday?
Ferb: Tell him your secret, and it may be the best birthday ever.