A mysterious girl arrives in Sunnydale between seasons 3 and 5 and starts making vast changes depending on which season it takes place in. This includes killing people (Ben, Riley, Angel, etc.). 

Because she’s killing seemingly innocent people, Buffy tries to stop her. But to her utter shock, she discovers that this girl is actually her daughter – and Spike is her father.

She's a Dhamphir; a half-human, half-vampire hybrid.

How the hell did that happen?

In season 6, Buffy’s various rounds of sex with Spike led to her getting pregnant. But Spike was killed in this future by a conspiring Giles and Angel, and Buffy’s friends made Buffy believe that Spike would have killed the child.

Buffy dies on patrol one night, leaving the daughter to be raised by Buffy’s prejudiced friends. The Scoobies tried to make the daughter believe their prejudiced views, but the daughter was able to figure things out for herself due to possessing special powers and strong magic capability.

The daughter fell in love with a vampire, but the vampire was killed by the Scoobies, which led to the daughter coming back in time in order to fix things.

If you want to go really dark, then Buffy refuses to believe her (swimming in the river of denial) until the daughter loses the will to live and kills herself, making a guilty Buffy realize what her and her friends’ denial and bigotry will cause.

The Scoobies must see the error of their ways in the end.

There should be an epiloge at the end which shows a happier future for them all.