It was a hot and sunny day in beautiful suburban Danville. Phineas and his stepbrother Ferb were sat under the tree in their backyard, trying to figure out to do today, the first day of summer.

"So, Ferb, what do you wanna do today?" Phineas asked eventually.

Ferb shrugged.

Phineas glanced down and happened to see his pet platypus. "What about Perry? What does he want to do?"

Perry made his usual chattering sound.

Phineas sighed. "Well, he's a platypus, they don't do much. 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, not like that one we rode at the state fair."

Phineas flashed back to the state fair rollercoaster. The car went over a tiny hill and before we knew it, the worker guy was saying, "Please exit to the left."

Phineas knew his stepbrother was also flashing back to that rollercoaster. "Man. That was lame. Why, if I built a roller coaster, I would…" He trailed off as he finally realised what would be awesome to do. "That's it! I know what we're gonna do today!"

The voice of Linda, Phineas and Ferb's mom, cut through the backyard. "Phineas, Ferb. I'm gonna pick up a few things. You boys stays out of trouble, okay?"

"Okay, Mom!" Phineas called back. He turned to Ferb. "We're gonna build a roller coaster!"

They got out their sketchpads and began sketching out blueprints. In the distance, they could hear some of Linda's conversation with their sister, Candace.

Candace: But what if there's an emergency?

Linda: Like what?

Candace: What if a satellite falls out of orbit and crashes into the house?

Linda: If that happens, you're in charge.

Candace: Yes!

Candace then entered the backyard. "Mom says I'm in charge. Conditionally."

"Whatever."

"Wait a minute, what are you doing?"

Phineas didn't want to deal with this now so he smoothly dismissed the question. "Homework."

"It's summer."

Phineas really wished she'd stop talking to them; they had a lot to do in not a lot of time. "That's cool. You wait till the last minute then."

"Well, I'm watching you. And I'm in charge. Conditionally!"

Phineas watched her go inside. "Let's invite Isabella over," he suggested to Ferb.

"Ah yes, your girlfriend. She would make a lovely addition to our rollercoaster-building team."

"I honestly thought that sentence was going to go a totally different way."

The blueprints done, Phineas and Ferb went and collected some wooden planks, steel beams, and some more stuff that they needed. By the time Candace stormed outside, they had a basic structure done. "Hey, check it out, Ferb. We can see Stacy's house from here, look!"

Phineas didn't actually notice Candace coming outside until she shouted, "Phineas, what is this?!"

"Do you like it?" Phineas asked.

Candace looked triumphant. "Ooh, I'm gonna tell Mom, and when she sees what you're doing, you are going down. Down! Down! Down! D-O-W-N, down!"

Phineas and Ferb stared blankly after her. "Do you have any idea what she's on about?" Phineas asked.

Ferb shook his head, just as Isabella walked into the backyard.

"Hey Phineas."

Phineas grinned. "Hey Isabella, my awesome girlfriend."

Isabella blushed. "Whatcha doin'?"

"Building a roller coaster," Phineas said, jumping down from the scaffolding.

Isabella came to meet him. "In your backyard?"

"Some of it."

"Wow. Isn't that kind of impossible?"

"Some might say."

Isabella looked up at the scaffolding and waved. "Hey, Ferb."

Ferb waved with the hammer.

"Does your step-brother ever talk?" Isabella asked, addressing Phineas, who shrugged.

"Ferb's more of a man of action."

Phineas and Isabella watched Ferb hit some nails in. As soon as they turned away, Ferb accidentally smacked himself in the nose with his hammer.

"Hey, Phineas? You wanna go swimming?

"We're kind of in the middle of something here, but if you wanna help us we can go later. If that's okay with you?"

Isabella grinned. "Okay! I love all your projects."

Phineas held her hand and walked over to the scaffold. "Hey Ferb! You got enough rivets up there?"

Ferb gave a thumbs up.

"What do you need help with?" Isabella asked.

"Okay, so the way I see it, the solid fuel rockets kick in at the mall parking lot," Phineas said. "Then we release the snakes during our corkscrew around the interstate. I'm gonna go get the snakes. Oh, and we're gonna need a blowtorch. Do you know where to get a blowtorch?"

"The hardware store will probably have blowtorches," Isabella replied. "I'm gonna go look there."

"Okay," said Phineas. "See you later."

Phineas had to make one last stop at a car factory. The manager came up to him and said, "Aren't you a little young to be a roller coaster engineer?"

Phineas grinned. "Yes, yes I am."

"Well, I must say, I'm very impressed. The forms all seem to be in order, although I've never seen them filled out in crayon before. So, if there's anything I can get you, anything at all, just let me know."

Phineas was only half-listening. He had spotted a robot arm that was putting a car door on a car body and welding it in place. "Do you think we could borrow one of those gadgets?"

Later, Phineas and Ferb and Isabella were riding the roller coaster with the arm on the front. The coaster was almost literally building itself.

"Now, this is the life," Phineas commented, taking Isabella's hand. Isabella grinned.

"Indeed it is. You should do all your projects like this."

"Where would be the fun in that?"

"We could spend more time together," said Isabella flirtatiously.

"Sounds good to me," Phineas said in the exact same way.

Ferb rolled his eyes. "Will you both just kiss already? Listening to you flirt is quite painful."

Grinning, Phineas and Isabella obliged.

Ferb went to go and put up some posters while Phineas and Isabella assembled the tent and stage.

Soon after this was done, there was a crowd of kids outside the tent. Ferb let the kids in and walked onto the stage and up to the microphone as if he was about to speak. He stood there for a second before he suddenly did a quick tap dance. Then he quickly stepped aside as Phineas came up from a trapdoor. Again, Phineas paused before grabbing the microphone and shouting into it. "Ladies and gentlemen, boys and girls, children of all ages! May I present to you a spectacle most of the morning in the making! Mark your calendars because you will want to tell your grandchildren about the day you rode the Coolest…Coaster…EVER!

Ferb pulled down the tarp to reveal the roller coaster. Everyone watched a bird fly into the construct.

"So, who wants to go first?" Phineas asked the crowd.

Everyone raised their hands.

On the rollercoaster, Phineas and Ferb were sitting at the front with Isabella and one of her Fireside Girls, Ginger, behind them. Phineas was sat up in the cart, showing everyone how to use their seatebelt, even though the rollercoaster was strangely seatbelt-free. "To fasten, insert the tab inside the metal buckle. To release, just pull back on the-." Phineas accidentally dropped the safety belt. "Oops. Well, you get the picture. Well, that's about it, enjoy the ride."

The car went over the top, and stopped, viewing the long drop. It was a VERY long drop.

"You guys all signed the waivers, right?"

That was the last thing everyone heard before WOOOOSH! The car sped down the hill and around the bends in the track. Everyone screamed, very loudly. Phineas and Ferb were the only ones not screaming, though they were enjoying the ride very much.

When the snakes dropped onto everyone, Phineas felt the need to say, "Relax, they're just rubber!"

The car went into a bucket of mud, then exited. Later, it went through a car wash and was cleaned from the mud.

Phineas suddenly spotted a familiar section of track. "Hey, look, there comes the AH-AH-AH-AH!"

The car sped over the piece of track that was going up and down repeatedly, creating four "AH"s.

The car corkscrewed around the interstate and did all the things that was in the blueprints, before Phineas looked back to say something to Isabella and spotted something familiar.

"Oh, there you are, Perry."

Perry chattered. Phineas didn't even question how Perry had gotten onto their moving rollercoaster. He supposed that Perry must have gotten on before the incline and nobody had noticed.

"Nice hat, Isabella," Phineas said, noticing a little fedora on her head.

He turned back. He didn't see Isabella take off the fedora and look at it, confused.

The car went to the end of the track and rolled into a construction area, where it eventually ended up getting flung into the air with a crane.

"Funny. I don't remember this in the blueprint," Phineas commented.

An airplane travelled by. The rollercoaster got caught on its tail. "And I'm sure this is new," Phineas added.

The coaster car dropped down and onto the Statue of Liberty, bending it and throwing the car toward Mount Rushmore. The roller coaster circled in Theodore Roosevelt's glasses before flying out and landing on a pine tree which bent down, lowering them down toward a Mr. Slushy Burger stand.

"Oh, hey, Jeremy."

The blonde-haired boy smiled. "Hey, Phineas. Anybody want fries?"

"Nah, we're good."

"Oh, and say hi to your sister for me."

The tree straightened out again and threw them to Paris in France. They got stuck on the Eiffel Tower just like they had with the tree.

"Crêpe, Suzette?" asked the man at the snack stall.

"What do you say, Suzette? You want a crêpe?" Phineas called back into the cart.

"No, thank you," a girl's voice answered. Phineas hadn't even known that there was a Suzette on the coaster.

The Eiffel Tower straightened out and the rollercoaster cart was flung into orbit. Everybody watched as a satellite flew by.

"You know, if that thing crashes into earth, Candace is in charge," Phineas commented to Isabella, who was clutching onto the rail in front of her like it was her lifeline, which it kind of was.

The cart suddenly fell down towards earth, towards the Tri-State area. The front of the car began to burn like a meteor.

"We should have charged more," Phineas commented to Isabella.

"You didn't charge at all, did you?"

"Nah. It's a figure of speech."

With a massive WHUMP sound, the rollercoaster car landed in the tree in Phineas and Ferb's backyard, depositing the two boys and Perry under the tree.

Linda poked her head round the gate. "Oh, hi boys."

"Hi Mom," Phineas called back, waving.

As soon as Candace and Linda left, kids began to drop down from the tree, commenting to Phineas as they left.

"Hey Phineas, that was great."

"Way too cool."

"That was awesome! Can we do it again?"

"Sorry, only one ride per customer," Phineas answered.

Isabella jumped down from the tree. Phineas caught her and put her back down on the ground. "That was great, Phineas. So, what are you gonna do tomorrow?"

"Don't know yet," Phineas said.

Isabella petted Perry. "Maybe you could teach Perry some tricks."

"Well, he is a platypus. They don't do much," observed Phineas.

"They're the only mammals to lay eggs," said Ferb.

Everyone stared at Perry. "…Maybe he'll lay an egg," Phineas said.

"Cool, see you tomorrow." Phineas and Isabella kissed briefly before Isabella went to the gate. "It really was the coolest coaster ever. You guys make a great team."

"Well, a brother is a brother, but I couldn't have asked for a better one than Ferb. You know what I mean?"

"Well, it was impressive. See you, guys."

Phineas watched as Isabella left the yard. "So, what should we do tomorrow? There's a world of possibilities. Maybe we should make a list!"

The coaster car exploded in the tree. A car alarm and a dog barking could be heard.