No, that's not true. I know what happened. I just don't know how it happened.

Actually, that's not true, either. I know how it happened. I was there, after all. What I don't know is why it happened.

I was walking down the hall towards the library. Free period means researching whatever demon's trying to destroy the world this week. I should really take the time to practice my guitar, but saving the world is part and parcel of being with Willow. I'm learning to enjoy it.

I never made it to the library. As I turned the corner by the science labs, an arm shot out and dragged me into the supply closet. I leaned against the wall as Xander closed the door and turned to look at me.

"Do I suddenly look like Cordelia?" I asked.

He shrugged. "She hasn't been joining me in here lately."

I raised an eyebrow.

Xander shook his head. "It's my own fault. I know that."

I nodded. I certainly wasn't going to disagree. "Then why are we here?" I glanced around to indicate our surroundings.

He shrugged again, sheepishly this time. "Actually, I wanted to apologize for the whole thing with Willow. Make it up to you, if I can."

Both my eyebrows went up on that one, and I just had to ask. "What exactly did you..."

Xander's lips on mine cut me off. I pulled back and opened my mouth in shock, and his tongue slithered in to curl against mine. I brought my hands up to his chest and tried to push him away, but he had me off balance. He shoved me back against the wall as his hand came down to squeeze my ass.

The kiss seemed to go on forever. After just a few seconds, I started moving, writhing my body against his. Xander pulled me closer and pushed his crotch against mine. I moaned into his mouth when I felt his hard length grinding into me.

Finally, after what felt like a million years, he broke the kiss. I groaned in disappointment and chased after his mouth, but stopped complaining when I felt it against my neck. He licked and nibbled his way down to my shoulder, pushing aside my shirt to get better access.

Something inside of me rebelled. Somehow, I found the strength to protest.

"We can't do this," I whimpered. Even to my own ears, it sounded pathetic.

"Yes, we can," Xander breathed against my skin, making me shiver in desire. "You want this as much as I do." It wasn't a question, and I knew he was right.

Still, I opened my mouth to deny it, then moaned when he sucked on my neck.

"I like that," I whispered. All thoughts of stopping were gone.

"Good," he said. "You're gonna like this even more."

I didn't know what he meant for a second, but then I felt his hands undoing my fly. For one brief moment, I knew I should stop him, no matter how badly I wanted him to continue.

"Willow..." I choked out.

"Willow will never know," he promised. "This is about you and me."

That was enough for me. I grabbed his head and dragged his lips up to mine, then gasped into his mouth as he yanked my jeans and boxers down to the middle of my thighs.

Suddenly, his hands were on me, pulling at my cock, caressing my balls. It felt so good I almost didn't hear Xander when he whispered in my ear.

"Where do you want my mouth?" he asked. "Up or down?"

"Down," I begged, and instantly he was on his knees. I looked down at him just in time to see him take me into his mouth. He raised his head to look me in the eyes as his tongue slid along the underside of my cock.

I bit my lip to keep from screaming. My hands flew down to Xander's head to run through his hair and push myself even deeper into his mouth. He choked for a second, then got control again, milking me with the muscles of his jaw.

I came, hard. I couldn't help it. Less than a minute in Xander's mouth and I lost all control. It was better than anything I'd ever felt before. I whimpered in ecstasy as I emptied myself down his throat. He sucked even harder, draining every last bit of me.

When I was completely spent, he let go of my cock with a wet sucking sound and stood up to kiss me again. It was softer this time, more tender, but just as exciting. I could taste myself in his mouth, and I licked away every drop I could find.

After just a moment, though, Xander pulled back to look at me.

"Are we even?" he asked with a smirk.

I had to catch my breath before answering. "Yeah, we're even," I told him with a weary nod.

"Good," he said, and slipped out the door.

I just stood there for awhile, trying to understand what had happened. Then I pulled up my pants, tidied myself up and headed for the library. There was still a world to save, after all.

I don't know what I'm going to do when I see Willow. I almost broke up with her for just kissing Xander, after all. How can I stay with her after this?

But I have to. I love her. I need her. She can't know about it, that's all there is to it. It happened, but now it's over. Like Xander said, she'll never know.

God, I don't even want to think about facing Xander again. Cross that bridge when I come to it. I have to deal with Willow first.

There she is. I just stop and stare through the library door for a second. She's so beautiful. I love her so much. I can't lose her.

I push through the doors and approach the table where she's sitting with Buffy. It's not until I'm almost there that I realize Xander is up in the stacks. I lose my stride for a second, but he just looks at me and gives me a small, secret smile. Maybe it'll be okay.

"Oz!" Willow cries out. She rushes over and throws her arms around me, and I know it really will be okay. It was just one time. It doesn't matter. It shouldn't have happened, but it'll never happen again.

"Hey, baby," I whisper to her, then kiss her gently. So gently. Nothing like the kisses Xander and I shared, but that's good. I don't want to kiss Willow like Xander kissed me. I love her too much. And she loves me. It's all good. I realize it's going to be all right as she gives a contented sigh and brings her head to rest on my shoulder.

"What's this?" she asks curiously. She pushes my head to one side. I go along with it, wondering what she's talking about. She's looking at my neck, at the same spot Xander was sucking on.

Oh, God, no...

"Oh, my God! Oz, who did that? I didn't do that!"

She backs away from me, her eyes horrified and unbelieving. She's looking at me, looking to me to explain this, to tell her it's not what she thinks. But it is what she thinks, and even though I'd lie to her if I could, I'd lie a thousand times to keep my Willow, I know she can read my eyes. Just like I can read hers.

I can see the exact moment her love for me dies.

"How could you?" she sobs, then turns away before I can see the tears start to fall. She runs out the door. Buffy looks confused, but runs after her, not even sparing me a glance.

I can feel eyes on me, though. I turn slowly and look up into Xander's face as he walks slowly down the stairs towards the door. He's smiling at me, but it's not like any smile I've ever seen on his face before. It's happy, yes, but also self-satisfied. Evil. Triumphant.

Without a word, he brushes past me and walks out the door. Going to comfort Willow, no doubt.

Now I get it. Now I know why.