Key

.

It had given her hope, once. Strength. That white envelope with the name "Elizabeth" on the outside. The TARDIS key for her sister.

Then Dawn had told her.

"Oh, yeah," said Dawn. "The Doctor got seriously worked up about it. He had no idea that he'd given it to me, and started going on about how I was going to survive and stuff. I guess that means it was… from his future-self, right?"

Buffy hadn't answered.

She went up to her room and shut the door, her back pressed against it. That key… not a symbol of hope. Just… one of causality. Of fate. Because the Doctor hadn't left that key for Dawn because she survived this. He'd left it because he remembered that Dawn had it in his own past. The key meant nothing. It gave Buffy no information about the future. None whatsoever.

Causality. Fate. Destiny. Causal chains that can't be broken. Patterns that can't be erased.

I'm going to kill your sister.

Buffy gazed out at her room, her back still against the door. Her eyes fell on the white envelope lying on her bedside table.

I'll have to do it. I'll have no other choice.

Her name written along the outside. No. Not her name. Other-her's name.

She tried to change the future. But by trying to change it, she made it happen.

The Doctor had seen Buffy's future, once. This time, she'd seen his. What he did next, after this year was over, after everything ended. He would travel back into her own past, into December of 1999, and beg her to kill him — justice, he said, revenge, he said, he deserved it, he said. Buffy had thought — she'd been so sure — that he'd watched her — Buffy Summers — die. But he hadn't, he hadn't, it hadn't been her

If I do the right thing, your sister dies.

One person dead. One person who'd fallen apart because of that death, tortured the Doctor, destroyed him, fed him to vampires, tried to convince him he was the most evil thing in the universe.

When you fight soulless creatures, you become soulless yourself.

Buffy knew the future. She could see it before her eyes, so very clearly. The Doctor would kill her sister. And she would destroy him. There was nothing she could do to stop it.

It had to happen.

Just like the Doctor had to give her that envelope.

Inevitability. Causal chains. Fixed points in time. Fate. Destiny. Doom.

In any timeline, in any set of circumstances, this is how we'll always wind up. As mortal enemies.

Buffy ran forwards, with an angry roar, and grabbed the envelope, ripping it into a thousand pieces. She shouted and screamed as she ripped those pieces into pieces, and then those pieces into pieces, and more and more until there was no envelope, no note, no nothing. Just a sprinkle of white paper.

Snowflakes in the California desert.

The next day, Buffy's mother died.