Summary: AU. It's a simple if/then formula. If he cheated on me with you, then he'll cheat on you... preferably, with me. This isn't a story about his romantic redemption but my ruthless revenge.

Note: this isn't all in Kagome's POV, there is third person narrative. You'll see. Also, Kagome is a bit OOC since I doubt that she'd be so vindictive but it's just a fanfic. Hope you enjoy.

Her "If/Then Formula" For Revenge

It shouldn't have surprised me like it did when I came home once, early from a business trip I had taken to Kyoto, to find him in bed with another. I had wanted to surprise him so I used the key he gave me to his apartment to bring him the souvenir I had gotten him while away — a brand new lingerie set, extra sexy. I look damn good in it, too bad he'll never get to enjoy it. Then … I heard the grunts, groans, and feminine moans. My mind lied to itself for my sake. When I was away he had to satisfy his needs, perhaps watch some porn.

That's all. That had to be all. The alternative wouldn't make sense … but then again, if I thought hard about it, it would. Sesshomaru did always say that monogamous relationships were more for mortals, that it went against a youkai's nature.

Anyway … when I got closer to the door that was already ajar, he wasn't alone. He was in bed … with you. That was the end of him and me.

If he couldn't stay faithful, then I didn't want him. It hurt like hell, yeah, but I could do better than him. So, I ended it that moment and threw the key he gave me at him. He tried to explain himself as he calmly came after me — because the asshole could never show passion for anything outside of his bed — but what could he say to make it better?

It always shocked me when I heard how a girl could so easily bed a man who was taken — either married or just dating. I mean, isn't it obvious? It's a simple if/then formula, we learn about those in high school. Anyone could prepare one, apparently you aren't smart enough to figure that out.

So, let me explain it to you. If he cheated on me with you, then he'll cheat on you. I guess I'm vindictive like that, but you were supposed to be my friend. Not the girl that slept with my boyfriend.

You're hardly appeared smart when I first met you, but I thought you had some redeeming qualities. You were kind of cut throat. I liked that about you. I should have known better than to introduce you to him. That was my fault. Apparently you were dumb enough to bed my boyfriend. I still can't really get over how stupid that is.

Sure, he's handsome, charming, and mysterious in that sexy way. I see the appeal, obviously since I was dating him, but seriously? He was mine. Not yours. Not up for grabs.

I was devastated for about a week, crying every once and a while and drowning my sorrows in a tub of ice cream. Yeah, the classic things that were awful for me and my health and that was when a real friend of mine gave me this idea. Sango, you knew her, she said to me that she'd make him pay for doing that to her and the bitch that screwed him.

I had asked her how, but she hadn't a clue. Killing him was an option, but we both felt that was a bit excessive. Then … I thought about it. The simple if/then formal and added a new part to the equation. If he cheated on me with you, then he'll cheat on you … preferably, with me.

That'd hurt you, I let it be for a while to see what you two were up to. I found out you started to date and that was when I put my plan into action. I can be so persuasive if I want to be, and I had been with him for over two years. I knew exactly what he'd like. It was easy to con him.

All I had to do was pick up a bottle of his favorite sake, put on that sexy red lingerie that I had bought in Kyoto with him in mind, put on a black wrap dress that he had always loved on me, slid on red stiletto heels and grab my long coat to head over to his.

I knew you wouldn't be there. It was a Tuesday night and he always had his important meetings on Wednesday. He never even let me sleep over on those nights. I doubt he'd let you.

When I knocked I knew he'd be able to smell it was me. If he opened the door, it would all work according to my plan, if he didn't then … well, it didn't matter. Obviously he opened it to look at me curiously, his thin golden gaze washed over me a couple of times before he asked what I was doing. I hadn't spoken to him since I caught him in bed with you. I get why he'd be confused.

I told him I wanted to talk, to make peace, we had been together for so long after all. Then I showed him the sake bottle, he eyed it shortly before stepping aside to allow me to enter his not so humble abode. When he went to get us glasses for the sake, I proceed to his bedroom.

Now would be a good time to pause and watch the DVD I sent you with this letter.

Her red eyes stopped then, it took effort, but she managed to pry them away from the handwritten letter to look at the DVD that had come in the same package. Kagura hadn't known what to think when she received it. It was clearly sent from Higurashi Kagome, but they hadn't spoken in so long….

It was obvious why, she didn't ever wonder. It didn't surprise her that she got transferred out of the advertising division — the one that Kagome was the boss of — the day after she was caught in bed with Sesshomaru. She couldn't fire her for that, just force her to work in the dull accounting area.

Kagura didn't care, she had Sesshomaru. Sexy, smart, full blooded youkai Sesshomaru. It was worth it.

Still, curiosity got her to open the package and find a long letter, pictures, and a DVD. The pictures … they made her heart stop and sink to her stomach. Her Sesshomaru was locking lips with Kagome, in bed with her, screwing her. But then she realized it was probably just a bluff.

Sesshomaru wouldn't cheat on her. She couldn't have cared less about Kagome's stupid if/then thing. They must have been photos from when they were together. That bitch was trying to trick her.

But then … what was the DVD? She took a deep breath as she inserted it into her computer to play it. Her hand shook as she pressed the "start."

"This thing better be working," Kagome whispered while she looked into the spy camera she bought for the occasion. A spy camera that she hid on his dress underneath one of his best jackets — one she had bought him. He was always awful about hanging it up.

She then walked away to sit on the bed, placing her hands behind her so she could lean back and cross some of her best assets — her long, lovely legs. At least three times a week she went to the gym to make sure they stayed such. While she was with him she had all the more motivation, he was gorgeous, so she had to stay that way to keep him.

Apparently, she hadn't been pretty enough.

"What are you doing?" Sesshomaru asked once he walked to his bedroom door, two glasses in his hand.

"Examining the scene of the crime," she replied.

"I knew that was what this was about," he said, withholding the need of a sigh as he walked in towards her. He set the glass that he had gotten for her down next to her while he took the bottle she offered him. "What do you want from me? To beg for your forgiveness?"

"I would never expect that from you," she said while she unbuttoned her jacket, slowly. It wasn't until she was certain she had his attention that she slid it off to reveal her sleeveless black dress that hugged her curves in all the right ways.

"Then what do you want?" he asked again as he poured himself a glass.

"Why do I have to want anything?" she questioned in return while she watched him take a drink. Personally, she detested the taste of sake so she didn't take any. Instead she picked up the glass and set it on the dresser, checking again to make sure the camera was angled towards the bed.

She was probably a horrible person for doing what she was about to, but … she had always heard revenge healed all wounds. Actually, she hadn't, but she thought it sounded good. Accurate, even.

"You know..." She turned to look at the man with perfect silver strands that sat on the edge of his bed. "You made a mistake. I doubt Kagura could be nearly as good in bed as I am."

"Ah, you've come to rub that in? To hurt me with that knowledge?" he guessed. "Why wait so long, then?"

"That's not what this is about," she said sternly while she walked up to him, grabbing he fabric tie to her side that kept her dress on her. "You're always so suspicious of people, Sesshomaru. You're like a wounded hound."

He took another drink, a long one that completed his glass. "Then, Kagome, what're you doing here?"

"I miss your touch," she said. She pulled the right piece to undo the whole knot. Her dress slowly fell off of her, all she had to do was move her shoulder slightly to allow it all to fall to the ground to leave her in nothing but a pair of heels and a lacey bra and panties.

"You want me to believe that you … want me back?"

"Everyone has their flaws. You made a mistake. One you won't make again."

"You sound so sure of that," he answered, giving in just a bit since he set his glass down on the floor and placed his hands on her hips.

"Oh, I'll make sure of that. After tonight, you'll realize you can't ever do better," she said before leaning down to press her lips to his.

It was then that Kagura couldn't take anymore, she shook her head … it was just a kiss. He wouldn't go any further. But just to make sure, she put the tape on fast-forward and watched as they got into bed together. What remained of either of their clothes fell to the floor before they became entangled under the sheets. It wasn't a onetime event, once they finished the first time she made sure they went for another round … and another … and another … until he was all warn down.

Kagome got out of bed while Sesshomaru was sleeping in it and put on her clothes and buttoned up her jacket. She walked back to the camera, Kagura played it at normal speed again. To see her lean in front of the camera, smile, and say, "It's simple, Kagura, if he cheated on me with you then he'll obviously cheat on you. I just proved it."

Then it went black … even though she saw it with her own eyes she couldn't believe it. She took a series of deep breaths before she picked up the letter again and scanned down to the part that she had stopped at.

I hope you enjoyed that DVD, Kagura. If you didn't watch it then I'll tell you want happened. Sesshomaru cheated on you, with me. You're an idiot if you think that'll be the last time he does it. He'll do it again and again and again.

You can't teach an old dog new tricks. I take pleasure in the knowledge that your career with Tokashi, Inc., won't go anywhere as long as I'm here. I suggest you cut your losses, dump his ass, and find a new job.

Goodbye forever, Kagura.

Ever so sincerely,

Higurashi Kagome

What she did was perfect revenge on Kagura, but she had only been half of the hurt. Sesshomaru had done the deed, having Kagura yell and break up with him wasn't really going to hurt him. Realizing that she had only slept with him that night to get back at him would perhaps irk him some, but he would still relish in the fact that they had sex.

So, she did the one thing she knew would piss him off to the ends of the earth … she went to his brother. The hanyou, Inuyasha. A cute man that lacked direction in life. They hated each other.

But each time she met him she could tell that he liked her … to use him merely to get back at Sesshomaru would be wrong. He was just an innocent bystander that had never been anything but kind to her. Regardless, she went to the garage he owned and waited in his office for him to show up.

He did good work fixing things that were so broken she didn't think they were salvageable. Like her car, a year ago, she got in an awful wreck. She had thought she'd have to get a new one but Sesshomaru — reluctantly — recommended Inuyasha. To her surprise, he managed to save it.

Surprise obviously made his golden gaze grow wide when he walked in. He didn't often have beautiful women waiting in his seat for him.

For a while he was silent and she just smiled until he asked, "Did you crash your car again?"

"Nope," she chimed.

That merely confused him again as he walked up to his desk. "I heard what Sesshomaru did. I could have told you that asshole would do something so stupid."

"I got back at Kagura, the women I caught him with," she assured him. He would find out eventually so she might as well tell him. It wouldn't be right to use him … not in less he agreed to it, that was. "It was kind of wrong of me … Do want to hear about it?"

"Sure." He sat on his desk next to her.

She bit her bottom lip before she looked up to meet his gaze. "I went to Sesshomaru's Tuesday night, videoed us having sex, and sent it to her with a letter explaining how stupid she was. After all, if he cheated on me, then he'd cheat on her. I just proved that."

He remained silent for a while before his brows bent together in confusion. "So … you're back with Sesshomaru? After what he did?"

"No," she laughed at the thought. "Of course not. It was just a means to an end."

"Ok ... so, what are you doing here?"

"I got back at Kagura, but I haven't gotten back at Sesshomaru. I would very much like to do that," she explained as she stood up in front of him. "So … I was hoping that you would help me with that."

"How?" he inquired with a lifted brow. "Do you want me to go beat him up or something?"

"No, no, not that," she assured him with a light laugh before she placed her hand on his thigh. "I was hoping you'd let me pretend to be your girlfriend. That'd really piss him off."

There was a fifty-fifty chance he'd take her up on that offer. She knew how the brothers feuded, so perhaps he would see it as another battle, or maybe he'd let his like for her get the best of him … or he would say no, be disgusted with her for thinking he'd let her use him in such a manner. The look on his face while he thought it over was so blank that there was no way for her to know which way he was leaning towards.

Then he finally said with a grin, "Why pretend?"

"You are aware I'm only here to make your brother mad," she said.

"Yeah," he nodded, "but I like bitches and you're a hot one. I think I can change your mind about this whole set-up. Maybe even make you want it to be a constant one."

"So you'll help me?" she questioned with a smile.

"You're a damaged," he had to point out, "but that's alright. I'm pretty good with broken things, too."

"This just must be my lucky day, then," she said.

Oh, how right she was … since there was little else she could have done to anger Sesshomaru than start up with his brother. A half-human brother, which begged the question of whether he, too, believed that monogamous relationships were impossible for a youkai. His answer was a simple one, and one she liked very much.

He was only a half-breed. A half-breed that disagreed with every single thing his brother said. Particularly his methodology of going about a relationship. That and he was frightened of what she'd do to him if he ever strayed to even think about it.

So, in the end, what she did was more than just get even. It was her own personal deterrence program at work.

A/N: I'm not one for the Sesshomaru/Kagome pairing, but a friend begged me to write a one-shot with them as the pairing so I did … probably not what she was expecting but she never specified that it had to be a happy romance story or even that they had to stay together or anything like that. Also, I've always wondered what anyone thinks when they enter into a relationship with someone who is already in a relationship. So I thought it'd make a good story.

I'm pretty sure any die-hard Sess/Kag lover would hate this story.

Thank you any one who reviews.