Candace and Stacy are gathering snacks for their viewing of a horror movie, The Grievance. Stacy leaves when Candace becomes more focused on what Phineas and Ferb are doing, which is planning a surprise for Isabella's birthday. 

After Perry enters his base, Major Monogram informs him of Doofenshmirtz's latest scheme. Doofenshmirtz has been seen driving around in a large truck equipped with a machine that turns objects into bugs. Perry leaves to stop him.

Back in the backyard, everybody continues planning for Isabella's party, just as she walks in and greets Phineas. He doesn't want her to see what is planned, so he has Buford take her away, which he does by putting her in a burlap sack. After he leaves, she explains to herself that all she wants for her birthday is some alone time with Phineas. Just after she gets home, she sees her birthday celebration.

Across town, Perry catches up with Doofenshmirtz on his "Bug Bus" but can't get to him due to his "Platypus-proof" bubble around him. He then explains to Perry about some things that bug him, such as portable outhouses and high voltage signs. Due to these aggravations, he created the "Bugs Me-inator". He explains that since he can't stand bugs, he made the bubble. Perry tricks him out of the bubble by showing him that a bug is in there with him. After Doof opens the bubble, Perry punches him, sending Doof swirling around. Doof then tries zapping Perry with his inator, but misses and hits his shoe. He then chases Perry around the top of the bus. He lunges at Perry, misses, and swirls around the bubble again. With no one at the wheel, the bus careens wildly around the town.

The Bus eventually crashes into Stacy's house, where she has been watching the movie. Scared from both the movie and the crash, Stacy notices Perry, and asks why he is wearing a hat. 

In Isabella's yard, the birthday celebration continues with the unveiling of a freakishly large red velvet cake, "with a special surprise". Isabella climbs up to the top of the cake, and makes her wish to spend time alone with Phineas. She blows out the candles, which shoots confetti fireworks into the air. Holly notices Candace waiting anxiously on her phone, and wonders what she is doing. Phineas asks if she got her wish, but Buford interjects, sarcastically saying that it was her wish to have her entire yard littered with confetti. Candace tells her that when things dangerous she will call Linda to bust Phineas and Ferb. Holly questions why she didn't call about the confetti, but Candace assures her that it will get worse. 

At Stacy's house, she wonders what is going on, and Doofenshmirtz explains to Stacy the basics of their routine. Perry tackles Doofenshmirtz, where Doof notices the cover of The Grievance on the floor. He says he saw it in the theater, and that he still has nightmares about the "greasy little girl who comes out of the basement". Stacy interrupts him, not wanting to know how it ends.

Next in the birthday celebration is a giant piņata which then splits into many smaller piņatas. After the piņatas are done, Phineas initiates the grand finale, a giant structure that rises out of the pool. The finale is unseen as Candace calls Linda to tell her to come across the street. As the guests awe at the "grand and glorious and potentially bustable gift", Candace runs across the street to grab Linda and take her to Isabella's house.

As Stacy tries to tell Perry and Doofenshmirtz to stop, she falls into the oil that was leaking onto the floor. She stands up, with oiled hair blocking her face. Doofenshmirtz sees Stacy and believes she is the "greasy Grievance girl from the basement" and flees in terror. As she asks Perry if the Flynn-Fletchers know about him fighting Doofenshmirtz, she steps on the Bugs Me-inator's trigger button. After if fires a beam, Perry destroys the -inator. The beam bounces off of a satellite, reflecting it back to Earth.

At the exact moment Candace is bringing Linda to Isabella's house, the beam comes into the backyard, where it turns the finale into a large amount of butterflies. Isabella is amazed, but Phineas and Ferb are confused as to what happened. Candace comes in and sees the gift is gone, now with butterflies in its place, and Linda thinks that the boys did this and loves it, wondering how long it took for them to collect them all.

Major Monogram, Carl, and several other people pull in front of Stacy's house, disguised as people from Danville Water and Power. Perry takes Stacy outside, and hands her the pamphlet explaining about host families and memory erasure. Not wanting to have Candace or the boys lose Perry, and also not wanting her memory erased due to not wanting to watch the movie again, Stacy exploits a technicality, saying that she is not part of his host family. She asks if it can be their secret. Perry thinks about it, then smiles and tips his fedora, agreeing with Stacy's wishes. To enforce this, Perry has Stacy lie to Monogram about not being home, which he buys. Perry and Stacy give each other a thumb's up.

All of the guests wish Isabella a happy birthday and leave her with Phineas and Ferb in front of her house. Phineas apologizes for the ruined finale, but Isabella consoles him saying that she loved the butterflies. To make it up to her, Phineas offers that if he and Ferb can take her out for some ice cream. As she was about to answer, Ferb lies that he has something that he needs to do, and that they should go. Phineas agrees, and Isabella fist bumps Ferb, thanking him for finally giving her some alone time with Phineas.

At the ice cream shop, Phineas again asks if she got her wish, but then remembers the confetti and streamers, actually believing Buford's comment. As he leaves the table to get more napkins, she watches endearingly, rests her head on her hands and silently says "And butterflies. You always give me butterflies...".

Phineas returned and "so Isabella in all did you enjoy your birthday." "yes yes I did Phineas but i'm disappointed that you didn't do more." "Well it's still your birthday  how can I make it up to you" "Well after were fished our ice creams we can go to see a movie back to my house and you can make it up me then." Afterwords back at Isabella's house in her room.

"Oh wow, Phineas that was a very good movie. I quite enjoyed myself." Isabella said, once they were up there. Phineas nodded. "Yeah, even though it was a romance flick." Isabella giggled at her boyfriend. "Oh Phineas, don't pretend you didn't like it." The red-head smirked at her. "I didn't. It wasn't true to life, the movie was corny. That kind of stuff doesn't happen in real life." Isabella was smirking back. "Oh yeah? Then what does happen in real life?"  The dark-haired young woman sat on her bed. Phineas moved closer to her and got on top of her, forcing Isabella to lay down on the bed. "Well, first of all," He began "There was no emotion at all in that movie." Phineas kissed Isabella passionately. The girl moaned and kissed back with the same fervor. The boy then removed his lips much to Isabella's chagrin. "Another thing is that the characters rushed into a relationship. Just because it's a movie doesn't mean it can't be realistic." Phineas then began to trail kisses down Isabella's jawline and neck, stopping at the junction of her neck and shoulder to gently suck at it. Isabella moaned again. Phineas lifted his head up. "A third thing is that the storyline was not true to life at all. It pretty much sucked. I think it was just an excuse to put sex in a movie." His girlfriend gave him a look. "Don't tell me you didn't enjoy that part." 

"Oh, I did. But I would have enjoyed it better if I could have enjoyed the whole movie." Phineas smiled.

Isabella smiled at back at him. "Well, could you show me how you would have liked it to go?" 

"I'd be happy to, but are you sure you want to Isabella?"

"Of course, you're the only one I want to do this with."

Phineas smiled broadly. "Well then, let tonight be the best night ever." He began to kiss her again, trailing kisses down her neck just like before. Suddenly, Isabella stopped him. "Phineas, we did that already. Let's move it along." "We can't rush these things you know." The red-head smirked. The noirette smirked back. "Okay, I know. I just-" Phineas put a finger to her mouth, shushing her. "I know, Izzy, I know. You'll just have to be patient."

Phineas resumed kissing her, this time trying reaching under her dress and rubbing her clit through her panties. Isabella gasped and moaned. She could feel his surprisingly soft hand slide the crotch of her panties over and insert a finger inside her wet pussy. She moaned again, arching her back at the pleasured sensation. No one else besides her has ever touched her there before and it felt great. 

"You're so wet Izzy." Phineas remarked, removing his finger to Isabella's obvious frustration. He noticed this and said "Don't worry, you'll be satisfied soon enough." Isabella then suddenly removed her dress, now only clad in her bra, panties and shoes. "I don't care if I'm rushing things, I was getting hot in that thing." She said. Phineas smirked. "Well, allow me to help you finish." He reached in back of her and undid the clasp on her bra. Once that was off, he removed her soaked panties and her shoes. Phineas took the time to admire Isabella laying naked on her bed. She looked so beautiful and sexy and it was making him hard. "I'm feeling hot too, Izzy." Phineas said, taking off his shirt. Isabella noticed the obvious tent in Phineas' pants. "Here, allow me to help you." She crawled forward and unfasted his pants, letting them drop to the ground. She then hooked her thumbs in the waistband of his briefs and pulled them down as well. Phineas stepped out of them and removed his own shoes and socks. Isabella took a hold of her boyfriend's erect cock and began gently pumping it. Phineas closed his eyes and let out a moan. Suddenly, he felt something warm and wet around his phallus and opened his eyes to see Isabella gently sucking on it. He groaned and tried to thrust into that delicious wet heat but Isabella kept him still with her hands on his waist.

All too suddenly, Phineas felt his member being met with cold air. He looked down at Isabella, who was rummaging in the pocket of her dress. "What are you looking for?" He asked her. "My strap-on. I always keep it on me for emergencies." Phineas stared at her questioningly. "You keep a strap-on dildo in your pocket?" "Yeah, ever since we started dating. I've always wanted to try this with you." Isabella smirked at him. Finding it, she put it on. It was your basic average skin colored dildo attached to black straps. "Now, get on the bed on all fours." She commanded. Phineas did as he was told. "Now what?" He asked. "Do you have any lube?" She asked. "Yeah, in my side table drawer." Isabella opened the drawer and dug through it until she found a small bottle of lube. She applied some to her hand which she promptly inserted a finger into Phineas's asshole. He gasped. "Isabella what are you doing?" "I'm going to fuck you with this dildo. I realize you aren't gay but there's plenty of straight couples who do this." "Uh, well okay if you say so." Phineas replied. Isabella continued to stretch him, adding more fingers as she went. Once she was able to fit three fingers in there, she lubricated the dildo and thrust inside Phineas. He let out a shout and dug his fingers into the covers below him. It hurt at first but soon felt good as he got used to it. Above him, Isabella was pounding into his tight ass. Phineas couldn't believe how good it felt. It was like nothing he ever felt before. Soon he began to moan and he screamed each time Isabella hit his prostate with the dildo. Eventually it all proved to be too much for him and he came all over the bed. Panting, Isabella pulled out and took off the strap-on. She laid down on the bed and spread her legs. "Now it's your turn." She said. Weakly, Phineas sat up and reached in the side table drawer to get a condom. Putting it on, he moved in between Isabella's legs. His cock was surprisingly still hard and once he worked up the energy, he slowly entered his girlfriend's virgin pussy. Isabella moaned which urged Phineas to continue when he reached her hymen. He broke through it and Isabella's face was mixed with pain and pleasure. Once he was sure that she wasn't in any pain, Phineas began to thrust in and out of her. Isabella moaned his name repeatedly and her pussy became wetter with each thrust. 

"Oh Phineas! Don't stop! Oh God! So good! Ahhh!" She moaned. "O-oh shit Izzy! You're so fucking tight! I-I think I'm gonna cum again!" Phineas panted. Isabella nodded and continued to moan. "Faster!" Phineas picked up the pace. The sound of Phineas's cock thrusting in and out of Isabella's wet cunt and the heavy scent of sex filled the air. Soon, both lovers couldn't hold back any longer and came together.

"PHINNEEAAS!" Isabella screamed in orgasm. Phineas only gasped as he came. He pulled out and took off the condom, tossing it away in his trash can. Laying down, Phineas pulled the covers over them and Isabella cuddled up to him and rested her head on his chest. They both sighed contentedly. "That was wonderful, Phineas. We'll have to do that again sometime." "Yes, yes we should Happy Birthday Isabella." Phineas replied. The two yawned before falling asleep in each other's arms.