In TV Guide, the Lost producers had their list of 5 episodes you should re-watch before the finale.
#100 - The Pilot
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Pretty obvious, this is where it all began.
#103 - White Rabbit
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This is the beginning of the "Faith vs. Reason" subplot with Jack and Locke, and Jack seeing his dead father.
#223 - Live Together, Die Alone
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Season 2 finale. This is when we first start to understand the island's strange properties. The Swan station explodes, an electromagnetic shockwave is released, crazy stuff happens, they see the four-toed statue, etc.
#322 - Through the Looking Glass
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Season 3 finale. We see the flash-forward with crazy Jack and Kate off the island, that all of Season 4 has been building up to.
#409 - The Shape of Things to Come
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We find out that the people on the freighter want to kill everybody on the island, and we see that once again Ben is not a man to be trifled with as takes control of the Smoke Monster to totally smash some faces, then in the flash-forward he kills some guys, recruits Sayid to kill some more guys, and then breaks into Charles Widmore's house and threatens to kill his daughter.
What are some other good episodes to watch? I'm thinking The 23rd Psalm with Mr. Eko, and The Man Behind the Curtain with Ben. And obviously, The Constant was an awesome episode.
i watched the white rabbit last night. i didn't think it was better than walkabout but it was still interesting.
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clearly something beyond understanding happened to Locke considering he can walk now, which makes me think there's more to the island than there seems