Like most TV dramas, Lost saves the big emotional moments for the latter half of the season, and Do No Harm stands out as the episode with the most at stake, featuring a series of situations that provide plenty of build-up for the season finale.
Picking up from the end of Deus Ex Machina, Boone is seriously wounded, with a collapsed lung and a crushed leg. With Locke missing and Jack not really believing his story regarding the origin of the injuries, tensions are already high when a race against the clock to save Boone's life begins. Meanwhile, Sayid and Shannon take their relationship further, and Claire goes into labor, causing part of the survivors to help her and Charlie to freak out.
Jack's determination to fix Boone is reflected in the flashbacks, which depict the events leading up to his wedding. It is revealed that the bride, Sarah (Julie Bowen, who has previously guest starred on another show with Matthew Fox, namely Party of Five), was originally a patient who many believed to be beyond help. Jack saved her, but is now uncertain when it comes to writing his vows. Help comes from an unlikely source: his father.
Although nearly every cast member gets something to do, the episode belongs to Somerhalder and Fox: the former finally gets a really big moment in the spotlight (Hearts and Minds notwithstanding), enhancing the script's emotional power with a compelling, vulnerable performance; the latter, benefiting from another solid chunk of back-story, emerges more prominently as the show's real human anchor, while also showing a tougher side with a deliciously bad-ass closing line that qualifies as one of the program's most mouth-watering cliffhangers.