A nice place to theorise on the upcoming episodes, anyways, here are the official summaries:

Episode 17 - Catch 22
Desmond coaxes Charlie, Hurley and Jin on a trek across the jungle after experiencing one of his future-prophesizing "flashes" -- but is he purposely placing Charlie's life in harm's way? Meanwhile, Kate turns to an unwitting Sawyer after seeing Jack alone with Juliet.


Episode 18 - D.O.C
After discovering that all of “The Others’” pregnant women died before giving birth on the island, an extremely reticent Sun allows Juliet to examine her -- and uncovers the identity of the unborn child’s father. Meanwhile, Desmond allows an unlikely nemesis to help save the life of a new, mysterious island inhabitant.

Episode 19 - The Brig
Locke breaks away from the Others to try to persuade Sawyer to help get rid of a common nemesis; a new island inhabitant discloses shocking information about the survivors' flight.

Episode 20 - The Man Behind The Curtain
No summary yet, but this is another reference to The Wizard Of Oz.

Episode 21 - Greatest Hits
Theorised that Charlie will die in this episode (also his flashback episode).

Episode 22/23 - Through The Looking Glass (Finale)
Jack's flashback episode. That's about all we know.