Not exactly the plot of Terminator, but loosely inspired by the idea/premise.

The story begins when Buffy is young. Between the ages 10-14; before she becomes the Slayer but old enough to understand dangerous and complex situations.

She’s having a normal day of her normal life when suddenly a demon tries to kill her. To her even greater surprise, another demon saves her, only this demon looks human; he has bleach blonde hair and wears a leather coat.

The demon – calling himself Spike – is able to kill the bad demon before he takes Buffy aside. He explains to her that she will one day be called as the Slayer; the protector of the world who fights evil. She will become the greatest Slayer that’s ever been, which is why the demon tried to kill her; it time-travelled (it could only do it once, so no more will be coming). Spike has time-travelled, too. 

Spike also tells her that they will meet again, only he’ll be evil and want to kill her. He explains that he changes, but it takes time, and this young Buffy can see that he means it.

Before Spike leaves, he gives her one final warning; to stay away from a vampire named Angel.

Since this is a young Buffy, like most kids, she wants her life to be less boring. She convinces her parents to let her take self-defence classes so she has some form of training before her official calling. She’s able to find information on vampires and demons, and learns that not all demons are bad.

By the time she’s called, Buffy feels that she’s ready to become the greatest Slayer there’s ever been.

Taking Future!Spike’s advice, Buffy stays away from Angel when she meets him. When she first meets Spike she naively tries to find his goodness, only for Spike – who is very evil and has no bloody clue who this Slayer thinks she is, telling him what to do – to still try and kill her. But remembering the kindness in Future!Spike’s eyes, Buffy doesn’t give up.

What else happens in the story is up to the author. Feel free to go completely off canon if you wish.