Although I rate this episode as one of my favorites, the entire reasoning for that is wrapped up in the final ten minutes when the sisters come together and realize the wrong that was done.
Up to that point, it is great, but still a very much average episode.
A simple one-line synopsis would be "A hard-hitting look into the future"
Phoebe (Alyssa Milano) gets a shocking glimpse of herself being burned at the stake in the year 2009 for a crime she states she did not commit.
However, when another glimpse reveals she did do the crime, she accepts her fate and burns for her crime.
As said at the top, the sisterly bonding before the burning is one of the best scenes in the history of the show.