Tons of action, lots of deaths, hope of rescue, and a big storytelling twist in the off-Island flashes of Jack...

Desmond bounces between 1996 and 2004 in a fascinating time travel adventure.

After turning the failsafe key in the Hatch, Desmond goes back in time to relive his breakup with Penny via a "lucid flashback."
http://lostpedia.wikia.com/wiki/Flashes_Before_Your_Eyes

Ben's arrival on the Island and some early history of the Dharma Initiative.

We (prematurely) find out what the future holds for Ben as he comes face to face with Charles Widmore.