The Apocalypse is near.
Michael and Lucifer are awaiting their Avatars - Dean and Sam Winchester - to accept this and them.
However, there are more Powers than angels and demons in the Universe.
Enter, Xander Harris. Having seen the portents of impending doom - and discussing them with Giles - he sets off on a quest to find someone powerful enough to oppose the Angels.
He meets and -eventually- falls in love with Illyria/Fred and vice versa.
This is set post season 7 so there could be a romantic triangle between him, Illyria, and whomever (Dawn, Faith, or Drusilla???).