When we left "Lost" in May, we learned that a strange dude - known to fans simply as "The Man in Black" (Titus Welliver) - is the sworn enemy of Jacob (Mark Pellegrino). He somehow has turned himself into John Locke (Terry O'Quinn), who was murdered by Ben Linus (Michael Emerson) and then managed to enter the Foot of the four-toed statue where Jacob has lived for . . . well, who knows how long. Locke /
Man in Black instructs Ben to kill Jacob. Before Jacob dies, he says, "They're coming."
Who "they" are is one of those mysteries that will/may be resolved shortly. We hope.
Meanwhile, back in 1977 . . . Jack (Matthew Fox) and Sayid (Naveen Andrews) have taken apart the nuclear bomb to get to the core. After accomplishing that, they rush back to the Swan Station with the rest of the gang - Hurley (Jorge Garcia), Jin (Daniel Dae Kim) and Miles (Ken Leung) - to drop it in the mine shaft.
Jack figures that by destroying the island, they can change the future - their future, and the world's future - but Sawyer (Josh Holloway) tries to stop him. But to no avail. Jack and everyone else arrive at the Station, where a ferocious gunfight ensues. The bomb is dropped in the shaft, but fails to detonate. Juliet (Elizabeth Mitchell) falls in. She bangs on it with a rock.