This is the last episode the Season 3’s final chapter: The Bloodline Chapter.
This episode will feature flashbacks to the night the Gilbert’s car drove off Wickery Bridge and Elena’s parents died.
This episode has the flashback closer to the present (Months before Pilot).

Continuity

Sara Canning will return as Jenna Sommers via Flashbacks.
Jenna was last seen in The Sun Also Rises, when she was staked by Klaus as the vampire sacrifice during the Sacrifice Ritual.

Behind the Scenes

The Departed is the title of the 2006 Academy Award-winning American action thriller film directed by Martin Scorsese.
Elena will finally make a choice between Damon and Stefan.
Lana Del Ray’s song Blue Jeans will be featured in flashbacks.[citation needed]
Filming began on March 23, and ended on April 2.[1]
According to Nina Dobrev a few people will die in this episode.
The title refers to all those who Elena has lost over the years, starting with her parents and continuing through Aunt Jenna.
A new pseudo villain will be introduced in this episode. [2]
Joseph Morgan has claimed that the Season 3 finale will be “epic”.
Candice Accola recently admitted that she cried reading the script for the finale.
This episode will have an underwater scene, featuring Elena Gilbert, Stefan Salvatore, Miranda Sommers-Gilbert and Grayson Gilbert.
We will see Damon telling Elena the first time he laid eyes on her knowing she wasn’t Katherine.
According to Ian Somerhalder (in a Spoiler Chat Daily article), we can expect some Delena goodness in the season finale and we will also see how much Damon really cares about Elena.
According to TV Guide, Ian Somerhalder has said, “What’s interesting about what’s happening right now is the dynamic between the brothers. They are starting to really bond. A tease about the episode coming up is really about these brothers overcoming their disdain for each other and ultimately realizing that family is really embedded into the fabric of who we are and if you can connect with that it’s really powerful.”
In an interview, Joseph Morgan said that the season 3 finale will shed some light on past events that happened in season 1, and Ian Somerhalder said something happens between Damon and Elena that he never thought of as Damon. [3]
There will be a massive, unexpected death in this episode.
When he spoke to TV Guide, he spilled that Damon and Elena (Nina Dobrev) are going to share a moment in the finale. “It’s ridiculously emotional … [the] scene with Damon and Elena and it was a very, very introspective scene that brought me back to the beginning of the show.” Co-star Joseph Morgan (Klaus) thinks viewers will get a sense of déjà vu as well, saying certain aspects of the episode will “shed some light on events that go way back to the beginning of the first season.”
Joseph Morgan said that the last 20 seconds of the finale with leave fans with their hearts in their mouth, and they will be wishing time will fast forward to season 4. He also said that there is so much shock, that the finale will change the world of The Vampire Diaries forever. http://blog.zap2it.com/frominsidethebox/2012/04/10-spoilers-a-vampire-diaries-cliffhanger—-a-james-van-der-beek-kiss-in-apartment-23—-and-more.html
Julie Plec said we will finally see who is behind The Darkness.
Stefan brings in an old ally.
According to TVLine, executive producer Julie Plec has said “confusion” is “the key word…for Elena.” She needs to figure out what she feels for each of them, and spending time with one and then the other does seem to be a pretty good way for her to do that. This is all leading up to her making a choice, and Plec says “the big thing with her choice is she’s not the one in complete control of it.” This is “really a decision about what does love mean to me and to us and where are we supposed to be in our lives with each other.” It does seem about time for her to make that decision, and considering everything that’s happened between her and each vampire, it would be surprising if she wasn’t confused. What will her decision mean for everyone in season 4?
According to TV Guide, Stefan and Damon are going to be trying to figure out how “to contain Klaus” since they can’t exactly just kill him. They don’t know if killing him would lead to their deaths since Kol killed the vampire who could at least give them a clue as to which bloodline they’re part of. Besides, they already know that killing Klaus would definitely kill Tyler, but as Caroline had told Tyler, Damon didn’t exactly care about that if he had found out he wasn’t in the hybrid’s bloodline. At least now it’s looking like it’s likely that Tyler will be safe, unless they get new information.
Interview - Julie Plec: The season finale — which will feature a flashback to the car accident that killed Elena’s parents, when Stefan first saw her and rescued her from the water below Wickery Bridge — will find Elena making a decision about her feelings for the Salvatore Brothers. “It will not be easy. It will be very emotional for both sides because it’s going to be a very honest, very pure, very difficult decision. Read more
-Vampire Diaries season finale kiss - “The Departed” will also feature “an important kiss.” Now, it’s already been revealed that Elena will be making her decision about her feelings for Damon and Stefan. Will the kiss be between her and the one she’s chosen? Could it be between Elena and the vampire she hasn’t chosen as a goodbye? It could always not even be between Elena and someone. Read more
In an interview with Zap2it, Plec confirmed that the episode will feature the car crash in which Elena’s parents died, but added that there will be no shocking supernatural twist on the event