Contains spoilers for this episode and earlier episodes.
Jack struggles to save Boone after Locke brings him to the caves with severe injuries. He initially works from Locke's claim that Boone fell from a cliff, but the damage does not fit that explanation. Boone needs blood, and Charlie cannot locate a suitable donor among the survivors he questions. Jack gives his own blood instead, using equipment improvised with Sun's help. The transfusion weakens Jack while Boone's crushed leg continues to endanger his life.
Sent to collect alcohol from Sawyer, Kate encounters Claire in labor and calls for help. Jin reaches them and goes to fetch Jack, but Jack cannot abandon Boone. He sends Charlie back with instructions so Kate can deliver the baby. Meanwhile, flashbacks show Jack preparing to marry Sarah, a former patient whose injuries he treated. Struggling to write his vows, he receives advice from Christian about commitment and his difficulty accepting that some things cannot be fixed.
Jack prepares to amputate Boone's damaged leg, but Boone regains consciousness and asks him to stop. He reveals that he and Locke discovered a hatch and were keeping it secret, then dies before completing a message for Shannon. Kate helps Claire deliver a healthy boy. Shannon, returning from an evening with Sayid, learns of Boone's death and grieves beside his body. Jack responds by going after Locke, convinced that the account he gave concealed responsibility for the fatal injuries.