A/N: Stinkfly3 here. This chapter should explain why Phineas and Ferb changed in the movie.
While most of this was going on, Ferb was in the backyard of his house, working on a new melody. He kept thinking about Isabella, the beautiful girl who changed his life.
Ferb sighed and leaned back against the tree. "I remember... It seems like only yesterday I became the person who I am today..." Ferb thought, recalling the events of August 5th.
August 5th
"Hey Ferb, where are you going?" Phineas called to Ferb as Candace had left them at the Flynn-Fletcher household while she muttered about having to go save her OTHER little brother and stepbrother. "Aren't you gonna play Doofopoly with me?"
"In a minute," Ferb said as he headed for his room. "I have some things to take care of."
While Phineas waited, Ferb got online and started browsing the web. There was something about Isabella that made him feel strange. It didn't take him long enough to realize that he was actually in love with her. But there was now a big problem: how can he win her heart?
"It seems like girls have a thing for male musicians," Ferb thought after searching through different websites thanks to Bing. "I think father has a guitar around here somewhere. I can teach myself how to play it."
But as Ferb was about to leave his room, he looked at his reflection in a mirror and realized that something was definitely wrong. "There's no way on Earth that Isabella would want to go out with me when I look like this!" Ferb thought as he looked at his reflection. "I definitely need to change my image."
After a few minutes, Ferb finally came back downstairs wearing a black shirt, black pants, black boots, an Army-green jacket, black gloves, and black sunglasses. However, Phineas did not recognize his stepbrother AT ALL.
"Who the heck are you?" Phineas asked the strange-looking boy.
"Your stepbrother..." Ferb said. "For like, forever..."
Phineas took a step back in surprise. "What on Earth are you wearing?"
"My new outfit," Ferb said with a grin. "I was thinking about changing my ensemble a bit."
"Well..." Phineas said. "I'm thinking your nuts. Put your Dooferalls back on before the Normbots see you. Don't you remember what Lord Doofenshmirtz said?"
"Let's just say I found someone else as my motivation," Ferb said, a picture of Isabella appearing in his head. "Now come on, let's find you something to wear."
"But...I'm fine with what I'm wearing," Phineas insisted. A sweatdrop came down the side of his head as he laughed nervously. "Honestly. Now let's forget this conversation ever happened and play another round of Doofopoly. What do you say? I...I'll...even let you spin first!"
"PHINEAS! Don't you want mum and father here right now?" Ferb asked. "I know how much you miss them."
Phineas looked at the family portrait and sighed. "Fine," he said. "But that doesn't mean I'll change."
Present
"Thank you Bella," Ferb thought, gazing at the lavender sky. "Thank you so much for giving me the chance to meet someone like you."