By now the story of Jin and Sun is approaching one of an epic love story, mainly in the amount of obstacles that have divided the two, even during the marriage. 'D.O.C.'s flashback takes place before the ones we've seen showing their marriage, reveal some stunning secrets about their families, including one that if Jin were ever to find out, he really might not be able to forgive.
All his life Jin has been ashamed of his family. We already know his father was a humble fisherman, and that he hid this before he began courting Sun. Which is why it came as something of a shock to learn (indirectly) in 'The Whole Truth' that Sun has always known about, and has been helping protect Jin. Now, in this flashback we learnt hat the truth was even uglier. Jin's mother was a prostitute, and his father--- the only compassionate parent we've met on this show--- even hid it from his son, though he raised him while he was never certain of the boy's parentage. Now decades later, an older woman claiming to be Jin's mother blackmails Sun by threatening to expose her role to her father.
Which leads us back to Mr. Paik, friend of humanity. Sun seems to have hid from her husband the evils that he did, but it was never clear that she knew. Now in her confrontation, she reveals she's always known, and it has sickened her as much as anybody else. Now she demands money to play the blackmailer, and the very act of protecting her new husband is what put him under Paik's thumb to begin with. He was working in a non-criminal capacity for him before, the money she asked for transferred the shame to her husband, who never wanted to be in debt to this man even now.
Up until now, Sun has had our sympathies in most of these flashbacks, but now that we know she's been holding back from her husband even more than he has, it takes a huge amount away. Before we could understand why she went into the arms of Jae Lee. Now it seems like she made her husband a monster, and then tried to run when she didn't like what she saw. And now Sun's secrets are coming back to bite her.
Perhaps it's not surprising that Sun is the person who calls Juliet on whatever the hell's she's doing, but enough her anger is genuine fright, which becomes even clearer when Juliet tells her that all pregnant women on the island eventually die. She then goes with Juliet back to a familiar place --- the Staff station, where Ethan kept Claire, and which Claire and Kate discovered a little later. This time, Juliet shows that there was useful material inside it-- including an ultrasound. Though the science that Juliet presents shows either that she's not the worlds best fertility specialist--- or that the writers got really lazy in researching their facts--- the scenes with her and Sun are pretty powerful anyway. Mainly because Sun is so frightened that she reveals a lot more of the reasons for her fear, and because Juliet genuine wants to give some good news after years of pronouncing death sentences when dealing with pregnancies. The irony is, she delivers one this time, but Sun takes it as a sign for rejoicing, because it's means the baby is Jin's , and that the island really did heal her marriage. Juliet seems to take it well, too--- until she goes back into the station, and records a message for Ben about the ultrasound she just performed. Even then, we still can't help but sympathize with her, especially after she turns off the recorder and says, no doubt in honesty: "I hate you." Still, this will have an effect on Juliet, and is probably the impetus for what happens next.
Meanwhile, on the other side of the island, Desmond's party is still trying to learn information from the woman who fell out of the sky. Problem is, she seems to be speaking in every language but English, and she's got a tree branch through her chest. Before they can figure out what to do, another figure emerges, and this is a bigger shock--- it's Mikhail. How exactly he survived the cerebral hemorrhage in 'Par Avion' is a question that has never been satisfactorily answered, and we certainly don't get one now, but they strike a Faustian deal in which he tries to save this woman's life. He does so, and then tries to steal the sat phone that she was carrying. Charlie doesn't want to, but Desmond allows Mikhail to escape---- an action that will end up costing them dearly in the end. This again, brings up the issue of which of the sides has done more to hurt the Other. Desmond brings up the fact that the Losties have killed five of the Others, while only two of their people have died. But all of the actions that the Losties have taken have been in reactions to ones that the Others have been made. When the final bell is rung by the end of the season, the factors will have changed far more, but it must be said that even now, everything the Losties have done can be justified, while the Others don't even try to defend their actions. Maybe they've been along so long they've got their own definition of what civilization means.
Eventually, the woman recovers and finally begins speaking English, and when she learns who these people are, she is shocked but not for the same reason they are--- Oceanic 815 has been recovered, and there were no survivors on board. Even now, it's still not clear what this woman knew about the flight, and what she was keeping from the survivors. But there is a lot more to this story that I don't think has been clear explained even a couple of seasons later.
'D.O.C.' ends with a moment of good news for one character, but one that also puts her in more definitive danger than anyone else on the island. Even if the Others don't come for her on Juliet's instruction, she will be dead some time in the next two months. This will reflect her actions, and as we know Sun is good at keeping secrets. But all her lies have mainly hurt her husband, and this one could lead to far worse consequences than the others.