"Morning, kids. Whatcha doin'?" Isabella asked her children on the first day of summer. "Well, nothing really, mom. We don't really know what to do," Summer replied, and Isabella took a second to think. "Well, Summer, I think it's about time I imparted the family legacy onto you," she eventually decided. Summer raised his eyebrow, and Isabella elaborated. "Phineas always did the most wonderful things during summer when he was your age… and I think his son should follow behind him!" she said. Summer followed behind with, "Then I know what we're going to do today!" Isabella wiped her eyes. Her son was really something special.

Vanessa woke up to find Ferb hastily writing notes. Not a good sign. "OK, what now?" she asked, wondering what it could possibly be. "A device to rust any metal to be unusable," Ferb replied. Vanessa smirked. "How predictable. I wonder how this is going to end," she muttered, staring at the place her father had taken up residence.

"MOM! Summer and Mist are building a rollercoaster!" Amanda yelled as she rushed inside. "Man, have I heard that one before. Jeremy, remember the time when… Phineas and Ferb… built… Amanda, we have to bust Summer and Mist to their mom!" Candace replied. Jeremy rolled his eyes. "Candace, aren't Phineas' kids a little young to be rollercoaster engineers?" he asked.

"Aren't you a little young to be rollercoaster engineers?" was asked to them by the person they wanted to buy parts to. Summer proudly stated, "Yes, yes I am!" as Isabella had told him, and the man replied, "Well, that's impressive. I've only seen one other young guy like you before, and he had these forms filled out in crayon. Well, is there anything I can do for you?" Summer pointed to the mechanical arm.

"Hiyo, Mist. Whatcha doin'?" Haley asked. "Haley? What are you doing here? School's out, you don't need to be writing that paper anymore!" Mist said, halfway towards gesturing to Perry. "I know, but I couldn't help but notice that behind you," Haley replied, pointing out the rollercoaster-to-be. "That's going to be a rollercoaster," Mist stated. Haley asked if he could be one of the first to ride it. "Mom?" she asked, and Isabella nodded.

Doofenshmirtz Evil Incorporated

"Ah, good morning Ferb! Have you seen the new rollercoaster? It's gonna be awesome!" Dr Doofenshmirtz said, before trapping Ferb in robotic arms. "I must say, I haven't seen my daughter. What's she up to?" the doctor asked, and Ferb shrugged. "Anyway, I've created a device that will rust anything I want it to!" he proclaimed, gesturing to it. Ferb's eyes widened. He had an inkling as to how this and his enjoyment of the rollercoaster were related, and he knew who was building the rollercoaster.

"Anyway, I'm going to the mall. You guys going to be OK?" Isabella asked. Summer nodded. "Haley?" she asked, and Haley responded with a thumbs up. Mist raised an eyebrow, but Isabella had left and Haley appeared to be deep in work.

"Aunt Isabella! You have to come! Summer and Mist are building a rollercoaster!" Amanda cried, trying to distract Isabella. "Amanda, what has your mother told you about spreading out-there stories in public?" she asked, hoping to deflect the accusation. "Nothing! She half-encourages it!" Amanda wailed, even trying to get the trolley out of Isabella's hands. "Amanda!" Isabella cried, and flung her off non-violently. "Now, if you'll excuse me…" Isabella said, and walked off.

Summer stepped up to the microphone. "And now, ladies and gentlemen, boys and girls, children of all ages… I present to you… THE COOLEST… COASTER… EVER!" he cried into the microphone. He gestured up to the now unveiled contraption, and everyone stared. "So, who wants to go first?" he asked, and everyone put their hands up. He decided on going with the Fireside Girls, Biff, Lloyd, and Daisy. He got everyone seated up, and pulled the coaster to the top of the first hill. He suddenly remembered what Isabella asked him. "You all signed the waivers, right?" he asked, before the coaster went downhill and no one got the chance to respond.

Ferb noticed a screw on the bottom of the floor. He wiggled his head, and head-butted it straight at Doofenshmirtz. He ducked, and chuckled. Ferb said, "Wait for it…" and the screw went flying… right into Doofenshmirtz' foot. He panicked, and rushed around the building, eventually getting the screw out, but horribly misaligning his rustinator. It fired, and… "Well… the insurance on this old building is going to take a huge hit… which is fine, because I'm technically squatting… RUN!" he yelled, releasing Ferb and rushing to evacuate… but Ferb was willing to duel with a collapsing rollercoaster threatening to destroy the lab.

"See, I told you, Aunt Isabella!" Amanda stated, pointing where the rollercoaster once was. Isabella gave her a deadpan stare, and went to go finish up the payments. Amanda went looking for the coaster… it was dangling from a helicopter. "AUNT! AUNT!" Amanda cried, trying to get Isabella back out.

Ferb launched a grappling hook at a passing helicopter, and launched out. Doofenshmirtz was left in panic as the rollercoaster fell entirely on his old apartment. Ferb climbed up to find Vanessa in the vehicle, in a perfect gambit. They suddenly changed direction, and lost some weight. They slowly turned… to see… a flying coaster.

Everyone started panicking as the coolest coaster ever went haywire. Summer started looking for an emergency control, but there wasn't one… just a brake. And brakes don't help an airborne coaster. After flying over America, they hit a temporary break in front of a Slushy Burger. "May I take your order?" the surly register asked. "Anyone want fries?" Summer asked, before they flew to France and had a similar discussion, this time with croissants. They entered orbit for a second… and came plummeting down. Summer quickly calculated angles and velocities and a whole bunch of other details before saying, "HOLD ON! SEATBELTS WON'T SAVE YOU!"

"See, Aunt Isabella! They aren't here!" Amanda cried, gesturing towards Isabella's backyard. Isabella rolled her eyes. "They are right there, Amanda. Now, I'm going to unpack the car," she said, leaving Amanda to stare at Summer and Mist. She went back home.

"Mist?" Haley asked, dropping from the tree. Biff, Lloyd, Daisy, and the Fireside Girls were unharmed, and India, Sam and Lara were half-wishing that they would have brought their brothers. But, of course, they were on official duties with the Fireside Girls… and their brothers might have missed it. Everyone but Summer, Mist and Haley left. "That was great, Mist!" he said, and Mist blushed. Despite designing the rollercoaster herself, she had gripped onto Haley in pure terror. "Hi kids! How was the rollercoaster?" Isabella asked, coming out into the yard. Summer said, "Awesome! We should do this more often!" and Isabella smiled. She blocked her ears, and the coaster exploded. "So what do you guys want to do tomorrow?" she asked, and Summer and Mist exchanged a shrug. "There's a whole world of possibilities, Mist! Maybe you could write a list of them! Anyway, I've gotta go. See you later…" Haley said, leaving the yard. Isabella crouched down beside Perry. "Maybe you could teach Perry some tricks?" she asked, stroking the pet platypus. "He's a platypus. They don't do much," Summer said. Mist pointed out that they are the only mammals to lay eggs. Isabella sighed, and headed back inside. Mist looked at Perry. "Maybe we should teach Perry something. From how Dad talks about his pet, he was much more than 'just a platypus'," she said.

Told you I had an idea for a sequel.

The goal for Hero's Legacy is seven chapters, for a full trilogy. I might do more if I make an error, but who wants to put down extra content? Anyway, as for who's kid is who's, I won't make a point out of outright stating it... but you should be able to occasionally figure it out by their names, characters, and other kinds of relations. It's not even important to the plot unless it is outright stated.

Anyway, carpe diem! Or some other line that Summer could use, I don't know.