Season 6, Episode 14:
The Candidate
Jack helps to rescue his friends from Widmore's clutches, the castaways enact their plan to escape the Island and the Man in Black's ultimate goal comes into view. In the flash-sideways universe, Jack tries to learn why Locke doesn't want to have his spine repaired.
Be warned: this one's a Wham! Episode and a Tear Jerker. Be sure to bring tissues.