Author's Note:

Hello everyone and welcome to my very first story! This may or may not be a series, depending on how I feel and how well it is received. I like to think this is a unique idea and although it is an AU, it'll follow the main timeline of the show, just with one major difference, which should be obvious from the title and summary.

Another thing is that I will be placing the age of Phineas and the gang at 9 years old, which seems a bit younger than most would put it (usually around 10-12). Although it is never explicitly stated in the series what age the gang is, Phineas and Ferb apply to college 10 years after the main show in Act Your Age. Average entry age is around 18 and one would assume that Phineas and his friends would've skipped a few years of high school given their summer activities. So the lowest probable age for them at the time of the show would be around 9. That's just my theory though.

It is also established that Isabella has been coming over to Phineas' house for eleven years up to Act Your Age (not counting the last 2 years up to the episode). That means she met him at around 6-7 years old.

Phineas and Ferb don't belong to me. They belong to Dan Povenmire and Swampy Marsh.

6-year-old Phineas was sitting in his room on his bed, absent from the family dinner, having dismissed himself early. He was eager to get back to his journal where lay endless drawings of inventions and projects yet come.

"Whatcha doin?"

Phineas jumped into the air as a girl appeared right next to him. His eyes widened when he took in the transparent girl sitting on the bed next to him. He began shivering slightly in fear.

"A-are you a ghost?"

The girl looked startled, "You know what a ghost is?"

"You don't?"

"No I…" the girl paused, "Nevermind. Sorry for startling you. And no I'm not a ghost."

Phineas had lost some of his early fright and was no looking at her curiously, "Who are you?"

"My name is Isabella," she responded with a giggle.

Phineas smiled and pointed at his chest.

"I'm Phineas!"

Isabella chuckled at his eager expression.

"I can't wait to tell Mom and Dad all about you!"

Her smile faltered, "I don't think they'll be able to see me Phineas. You're the only one who's been able to so far."

"Then you're my special friend. Only mine." Phineas didn't appear too fazed.

"Special friend." Isabella's face grew radiant, "I like that."

7-year-old Phineas and Isabella were standing in the garage, looking over the array of tools and materials in the room. Mom and dad had taken Ferb to a dentist appointment and Isabella had invited Phineas to explore.

Currently, he was scratching his head, "How does this bring my projects to life?"

"You just have to believe in what you imagined Phineas," Isabella said excitedly, "Carpe Diem."

Phineas looked at her strangely, "car-pie-dee-em?"

Isabella rolled her eyes, "Carpe diem, it means seize the day. You should try it!"

Phineas brightened and made his way to the closest pile of tools. He tried to lift the box, but almost dropped it due to it's weight. He let out a groan as Isabella laughed at him.

"You're not very helpful are you?"

"Normally I would," Isabella shrugged as she held out her ghostly hands, "but you know…"

Phineas sighed as he gently returned the box to it's position on the shelf, "Maybe another time. In a couple of years."

8-year-old Phineas and Isabella sat together under the tree. Well Phineas sat while Isabella just floated in a sitting position. To any other observer, it would appear that he was talking to thin air; but to Phineas, he saw his best friend sitting beside him, smiling at the expression on his face.

"That's funny," Phineas chuckled, "I can just imagine the thrill."

Isabella smiled at him, her hands folded in her lap and her pony tails tucked behind both ears.

"Yeah, rollercoasters are a fun concept aren't they," she responded shyly, "I've always wanted to ride one."

"Well, mom and dad are planning a trip later this afternoon, so why can't you come with us."

Isabella's face faltered and a flicker of sadness crossed her face, "I won't be able to enjoy anything with you Phineas. I'm not real remember?"

"You are real," Phineas insisted while pretending to place his hands over her own. This elicited a faint shade of red to appear on her cheeks, "You have feelings and thoughts. You taught me to enjoy summer to it's fullest. You were family to me as much as mom and dad, or even Ferb."

Isabella was blushing fully now, even if young Phineas didn't understand it. "That's sweet Phineas, but one day, you're going to get older and I can't be around forever to orientate your life."

"Now that's a depressing thought," Phineas chuckled slightly, "I can't imagine life without you."

Isabella smiled once more, but it was a sad smile now. "Ferb is coming back soon. I should go. Carpe Diem Phineas."

Phineas waved slightly as Isabella faded away. Ferb walked through the gate and found Phineas with his hand up mid-wave.

"Isabella?" he asked simply.

Phineas smiled at his brother. Ferb was one of the few people who believed in his friendship with the imaginary girl.

"She was looking forward to going with us."

Ferb raised an eyebrow and Phineas sighed sadly.

"I know that's not possible, but sometimes I really wish it were."

Ferb patted him on the shoulder softly.

9-year-old Phineas was sitting under the tree thinking next to his brother Ferb, pondering how to kickstart their summer vacation.

"…I for one, am starting to get bored, and boredom is something of up with which I will not put! The first thing they're gonna ask us when we get back to school is 'what did we do over the summer?'. I mean, no school for three months. Our life should be a roller coaster! And I mean a good roller coaster…"

Phineas trailed off as Ferb looked at his brother knowingly. Dozens of memories of sleepy mornings, lively afternoons, and quiet nights with a girl dashed through his mind. Memories of a girl who gave him a purpose, a motivation, and most importantly gave him love.

Carpe Diem, Phineas mused.

"That's it! I know what we're going to do today!"

A glowing smile crossed Phineas' face.

"We're going to build a rollercoaster"

And cut! This story is going to run parallel to the show, with a few exceptions. Eventually, there will be a few standalone chapters, specials, and such. There will probably be a finale as well depending on how long I want the story to be.

Less emphasis will be placed on the Perry/Doofenshrmitz or Candace subplot since the focus will be Phineas and Isabella. I won't be doing all the episodes either, maybe just 1 in 3.