final episode of season 5
"The Incident" May 13, 2009
In 1977, the survivors succeed in detonating the modified nuclear bomb at the construction site of the Swan station, the finale ending when the bomb is detonated. In 2007, Locke and the Others travel to the base of the four-toed statue, where Jacob lives. Locke is revealed to be an impostor: an old acquaintance of Jacob who manipulates Ben into killing Jacob. In flashbacks, Jacob visits several of the main characters.
from what i've read, season 6 starts with the plane landing in LA as if none of the last 5 seasons ever happened
it is said to be the last season and the producers have said they will wrap up the story and it will make sense