Buffy: The Vampire Slayer Season Five
The dreams at the end of season four prove to be portents of the events of season five. Season four was also a furthering of the experiment of humor, which season five carries on from the get-go. The first episode finally pits Buffy up against the legendary Dracula. Xander is controlled by the Dark Prince as the Renfield of the tale. Giles is overcome by the Three Brides, and Buffy is shown a taste of the dark side. The episode ends with the arrival of Buffy's sister. The season is spent explaining her existence.
The major evil of season five is a God from a powerful triumvirate in a hell dimension. Glorificus (or Glory if you prefer) was usurped and forced into a mortal body on Earth. She was literally so terrible that the other two gods were frightened enough to stage a coup. Ever since, Glory (or the Beast), has been trying to locate a key back into her home dimension. The problem is that the key was hidden from Glory by some industrious monks, and they could have stuffed it into any form.
It becomes apparent early on (especially to Buffy aficionados) that Dawn is the key. And that she's very annoying (although that doesn't correlate to her being a key). Due to Dawn's connection with Buffy (the monks used the slayer's blood to create her), everyone still treats Dawn like a little girl. The season continues in the vein of humor, but quickly focuses more on relationships.
Almost every relationship on the show is tested or changes, Buffy and Dawn as sisters, Tara and Willow as lovers, Xander and Anya as a serious couple. Buffy and Giles strengthen their bond, and Spike spirals deep into his obsession with Buffy. Finally, everyone's emotions are drawn to a breaking point when Joyce (Buffy's mom) suddenly dies from complications with cancer.
Joss had received word that The WB was canceling the series, so he ended it dramatically with Buffy jumping to her death to save her sister. Season five is a roller coaster of drama, comedy and grief. It's completely worth the annoying Dawn moments.