Lost (TV series) - Wkp - Season synopses:

Season 1 ...The surviving passengers of Oceanic Flight 815 are threatened by mysterious entities including polar bears, an unseen creature that roams the jungle, and the island's malevolent inhabitants known as the "Others". They encounter a Frenchwoman Rousseau who was shipwrecked on the island over 16 years earlier and find a mysterious metal hatch buried in the ground. An attempt is made to leave the island on a raft.

Season 2 ...The hatch is explored and the existence of The DHARMA Initiative and its benefactor, the Hanso Foundation, are revealed. New characters are introduced, including the tail-section survivors and other island inhabitants who that is not a tailie or an Other? only Desmond, in the hatch. There is growing conflict between the survivors and the Others ~they have Walt and survivors have HenryGale, with the continued clash between faith and science being thematic in certain episodes. As the truth about the mysterious Others begins to unfold, one of the crash survivors betrays the other castaways Michael helps the Others capture Jack, Kate, Sawyer; in return Michael & Walt are given a boat by wh to leave the island, and the cause of the plane crash is revealed.*

Season 3 ... New crash survivors really? new to us, not to their fellow survivors, right? yes Paolo & Nikki and Others are introduced. One of the Others Juliette and a new island inhabitant (Naomi ?she is not there very long ~ 'inhabitant') no Desmond join the survivors while a survivor Locke (I think - this happens earlier - in finale he is back w survivors, I guess after Ben shoots him ~ bcs his leadership is threatened by Locke, who was able to hear Jacob ~) defects to the Others. A war between the Others and the survivors comes to a head, and the survivors make contact with a rescue team.

DLindelof (I think) said: Season 1 is introductions; 2 is into the hatch; 3 is with the Others.
and I guess now season 4 is 'the beginning of the end' ~ the freighter, helicopter four, Abaddon, wreckage of 815, Oceanic 6.


*Oceanic Flight 815 - Lostpedia:
In "Live Together, Die Alone" Desmond tells Locke that he crashed the plane. After accidentally killing Kelvin Inman, Desmond returned to the Swan having not entered the Numbers into the computer in time. A system failure sequence began, and metal objects were pulled around the room. After managing to enter the Numbers, the alarm died down and the station returned to normal. On the same day, Oceanic Flight 815 crashed on the Island.
In the Season 3 special feature "Access: Granted" Lost writers Damon Lindelof and Carlton Cuse explain that Desmond's failure to push the button caused electromagnetic forces to build up which interfered with the plane's instrumentation, causing it to crash on the Island.