Quite possibly one of the top 5 best episodes of Lost, and definitely the single greates scene in the whole show(the only one that comes close is the one in the Walkabout episode, when we discover Locke's past accompanied by the legendary words "Don't tell me what I can't do!") where Jacob tells Richard what the island is and we lear how Richard got on the island, and more importantly how he became immortal.
This is every reason why Lost was so damn great.
Review of Ab Aeterno
