Disclaimer: I don't own Phineas and Ferb, or any of its characters; all credit to Dan Povenmire and Jeff "Swampy" Marsh. I only own this plot.

Phineas and Isabella are 18 and living together, but they haven't gotten too serious yet. Well, that's about to change. Enjoy!

Phineas stood at the foot of the bed, waiting for Isabella to come in. They had moved in together two months prior, and had only started sharing a bed a month ago. Phineas had been nervous about it, but found himself rather enjoying it.

Isabella walked in, wearing her usual nightwear; a pair of black shorts and a light pink tank top. Phineas was in a grey t-shirt and navy shorts as per usual. She smiled at her boyfriend and walked up to him.

"You ready for bed?" asked Phineas. Isabella thought for a moment, and then answered him with another question.

"Phineas, how tired are you right now?" she inquired. He raised an eyebrow curiously, but answered.

"Not very. Why?"

Isabella nodded and took another step forward, placing a hand on his chest. Then she smirked and pushed him back down on the bed, making him let out a small sound of surprise. He tried to get up on his elbows but Isabella climbed on top of him and held him down.

"Wh-what are you doing?" he stuttered. She smiled playfully and reached down to his waist-line, pulling his shirt up and over his head and throwing it aside.

"Just having some fun," she said. "Now close your eyes and wait."

Phineas had no idea what was going on, but he did as she said, lying there with his eyes closed. He felt Isabella get off of him and heard the sound of a drawer opening. In an instant she was back on top of him, and he felt her grab his right wrist and bring it up above his head. He felt a strange pressure on it, and heard a metallic clicking sound, and then she let go, but the pressure was still there.

"Alright, you can open your eyes now," she said. Phineas did, but when he tried to pull his hand down he couldn't. He looked up to see that he was now held to the bed with a pair of handcuffs, one around his right wrist and the other attached to the bed-frame.

"You handcuffed me to the bed?" he said in disbelief. Isabella smirked again.

"I'm just making sure you don't go anywhere."

"Why do you even have handcuffs?" he asked.

"Buford lent them to me," she explained. "He has dozens of extras."

With that Isabella leaned her head down and kissed him. Phineas was hesitant at first, but eventually he got into it more. Isabella's tongue licked his lips teasingly. When she pulled away, Phineas was still helpless to do anything. He watched with wide eyes as she straddled him and put her hands to the bottom of her shirt, starting to lift it up. Phineas must have made a sound because Isabella stopped and raised an eyebrow.

"Would you like to do it instead?" she asked, to his extreme embarrassment. Phineas struggled to get any words out.

"I uhh, don't think I can," he managed to say, shaking his wrist slightly and making the handcuff click.

"Good point," said Isabella, and then she pulled her shirt all the way over her head. She tossed it to the side as Phineas found himself staring out her now uncovered chest. He felt a sudden rush of emotions and may have been able to get something out, but Isabella leaned down again and kissed his lips briefly, then going down farther and kissing his chest. Phineas closed him eyes again and moaned slightly, his free hand tracing over her curves. Isabella broke away for a moment and lifted her head up.

"You're not going to get mad at me then?" she asked quietly. Phineas dragged his gaze away from her chest long enough to shake his head, still not able to get any words out. She smiled mischievously and traced her hand over his chest, slowly moving it down until she reached his shorts. She moved herself down and slowly started sliding them off of him. Phineas closed his eyes again and let Isabella do what she wanted. Not that he had much of a choice anyways. He felt his shorts slide down his ankles, and then felt them fall to the ground. Isabella leaned down and kissed just above his waist.

"You are such a tease," he whispered, his eyes still closed. He heard her laugh, and she slowly brought her lips lower. He shivered as her tongue flicked over the tip.

"I-Isabella," he managed to get out. She lifted her head and raised an eyebrow at him. Phineas slowly opened his eyes and looked at her.

"I'm not going to try and run," he said, although he still stuttered slightly. "You can take the handcuffs off."

"No way," she replied. "It makes it all the more fun for me."

Phineas rolled his eyes, and she winked, bringing her head up to his and kissing him hard on the lips. He kissed her back, trailing his free hand up her stomach. He paused for a second, and then brought it up higher, his fingers just barely touching her left breast. She responded by slowly sliding her tongue into Phineas' mouth in their kissing, teasing him even more. Her hands slid down his chest, below his waist. He moaned and kissed her harder.

Suddenly she lifted up and removed her hands. Phineas frowned in confusion, while she only smirked.

"Done," she said simply, getting off from on top of him. He watched as she got herself onto her side of the bed, lying with her back to him.

"Are you going to let me go now?" he asked, surprised at her sudden stop.

"Yeah right," she responded, her voice muffled by the sheets. Phineas raised an eyebrow. She was devious.

A few hours later, Isabella awoke. It was dark, meaning it was still night. Her eyes took a moment to adjust. She tried to sit up but felt resistance on her right arm. She looked up and to her complete surprise, found her wrist now handcuffed to the bed. She pulled on it, but to no avail. She heard a quiet chuckle and turned her head to see Phineas lying there on his side, watching her with an amused smile on his face. He was free from the handcuffs, and he had his shorts back on.

"Phineas, what is this?' she asked in confusion. He sat up and carefully climbed over top of her, his face hovering just inches from hers.

"Simple," he whispered. "You had your fun; now it's my turn."

Before she could respond he leaned down to kiss her, soft at first, but slowly getting more passionate. Isabella traced her free hand over his chest, not caring any more that she was handcuffed. Phineas' hands moved slowly down her sides, only pausing briefly at her waist. Then he placed his hands down and slowly slid her shorts and underwear down in one go. Isabella let out a small sound of surprise and blushed, but she didn't protest. Her blush only deepened as Phineas slowly moved his hands over her body, trailing over a very sensitive area. He broke away from their kiss briefly to move lower, kissing down her body, pausing at her chest, making her shiver with anticipation. She closed and eyes and leaned her head back in bliss as he slowly licked over her nipples, no doubt trying to tease her, yet she still loved it.

Phineas placed his hand down on the sensitive area and slid his index finger down. Isabella shivered and her breathing got heavier as he slowly slid his finger in. She let out a small gasp as he started to move it in and out at a steady pace. She kept letting out small sounds of pleasure. Phineas continued his teasing kissing, never staying in one place for more than a few seconds before moving on somewhere else. Isabella was feeling slightly surprised at his initiative, but she was definitely not disappointed. Her breathing got heavier as Phineas moved faster, adding another finger and making her moan in pleasure.

"Oh god Phineas," she moaned, her back arching unconsciously. Phineas smiled and went back up to kiss her on the lips again. His other hand trailed up her stomach, stopping just before her chest and then going back down. The teasing was driving Isabella crazy. Then, just as suddenly as she had, Phineas stopped and removed himself from her. She opened her eyes to see him lay down next to her, an obvious smile on his face.

"Phineas, would you please let me go now?" she asked sweetly, in a voice she knew he couldn't resist. He rolled his eyes in fake annoyance and complied, removing the handcuffs and setting them down on the bedside table. He was just about to close his eyes to sleep when Isabella giggled and pinned him down to the bed. He laughed, and she leaned down to kiss him again.

This was going to be a loooong night.

Let me know what you thought!