Note: This story actually happened in a dream I had last night. I am simply telling it the way it happened.
Phineas and Ferb decided to have a motocross race. Like always, they had it all set up within about an hour. But NOT like always, Phineas insisted on finishing the bikes while Ferb went to gather their friends. The race went pretty well. At one point, first-place Isabella came up to the last place racer and said, "Hey Phineas," in a slightly mocking voice, "I'm about to lap you." But sadly for Isabella, Phineas sped up and would not allow his friend to pass him again.
At the end of the race, Isabella was way out in front of everyone. She saw the finish line and started prematurely celebrating. I say prematurely, because at that very moment, her bike started to sputter and die. She was out of gas! She laid on the gas for all it was worth, but ended up just short of the finish line. She then turned around and watched Ferb, Buford and Baljeet's bikes all suffer the same fate. Then Phineas came up behind everyone, swerved through the stopped bikes, and won the race. He did a full 180 on the ground, and started laughing. "I can't believe that worked!" he exclaimed. "I rigged your bikes to run out of gas right before the finish line. And none of you suspected a thing!" Phineas looked at his friends nervously, having just realized what he had done. Luckily, they all just laughed it off.
The kids started walking away from the racetrack, which had inexplicably been leveled and turned into a farmer's field. But nobody noticed. Instead, Isabella noticed that the sun was beating down on her, from directly overhead. That meant it must be only about noon. "So Phineas," she asked, "what'cha doooooin' after this?"
"Well, I guess... I don't know," he replied, clearly caught off guard. "I mean, usually our projects take up most of the day. I didn't really... have any plans."
"So... you wanna... come over for a swim?"
"Sure!"
Cut to the Doofenshmirtz and Agent P storyline, which didn't appear in this dream. Cut back to Phineas and Isabella walking towards her pool.
"I still can't believe you managed to pull off that prank," Isabella said, half flirtatiously and half mischievously.
"Yeah, well, I guess I'm just lucky nobody took it the wrong way. I'm glad I didn't get any comeuppance."
"Oh really? You mean like... this?"
Suddenly, Isabella tackled Phineas, held him on the ground, and started tickling him. Phineas realized that he had been played. He was very ticklish. He wondered if Isabella was as ticklish as he was. He just had to wait for the opportunity to find out.
At some point, Isabella stopped tickling, apparently having decided that Phineas had had enough. "Was this your plan all along?" he asked. "Did you invite me over here just to tickle me?"
"Yep!" Isabella said with a smug grin on her face. "And it was fun!"
"Oh, it was fun?" He had the perfect opportunity now. "So I guess you wouldn't mind if I... got in on the fun?"
And just like that, Phineas rolled over and started tickling Isabella. She had been right; it was fun. But he barely had time to process this thought before Isabella rolled him over and tickled him again.
This continued for a few minutes. Every few seconds, the two would roll over and take turns tickling each other. Then, Phineas rolled over onto Isabella... and watched her drop into the pool. Thanks to the law of gravity, Phineas followed suit shortly afterwards. So they let go of each other and swam to the surface. When they came back up, they both looked at each other with slightly mischievous grins on their faces, and the tickle fight continued.
At this point I woke up, so I have no idea who won. But I would love to hear your thoughts on the matter.