In an episode that's almost entirely flashback, we find out (sorta) what happened to Desmond after he turned his key. After Desmond saves Claire from drowning, Hurley's more convinced than ever that Des can see the future (or in this case, the present), and he and Charlie take it upon themselves to find out what's up.
Figuring that the way to a Scotsman's heart is through his liver, they get him drunk. Which has the, um, unexpected result of making him belligerent and combative. But we learn that when he was knocked out by the hatch explosion or implosion or whatever, he flashes back over a good chunk of his life, from Penny moving into his apartment to Desmond breaking up with her. But he seems to be reliving it, with flashes from his island life. I guess this could be chalked up to all this happening while he was unconscious (and more of a dream than anything else), except after he woke up naked in the jungle, the future flashes keep happening. Which is how he knew in advance about Locke's speech, about the lightning that was going to strike Claire's hut, and how he knew Claire was in trouble in the water.
While back in his life in England, he gets all up in The Matrix with an elderly woman filling him in on how no matter how he tries to fight the future, the universe has a way of correcting course. So even though he knows he shouldn't go on the boat race, he's going to wind up going anyway, and even though he knows he should stay with Penny, he doesn't feel he can, which is the most convoluted "it's not you, it's me" ever.
So his odd behaviour on the island is now about him trying to change the future to prevent bad stuff from happening. Which is such a compelling idea that a show ABC tried to fill Lost's spot with, Black Groundhog Day, which had THAT EXACT PREMISE, tanked after only a few episodes (but you can watch it online. Awesome!)
And get this: Desmond's been trying to save Charlie. Because the universe wants to kill Charlie. Hee! My question: since Desmond knows the future can't be changed, why's he knocking himself out? The universe will have its way. Course-correcting, dude. Sorry, Charlie.
Desmond strolls along the beach and notices Hurley and Charlie, who are over in Sawyer's tent, going through his things. There's a stack of books, the top one of which is a Vladimir Nabokov, in those awesome Vintage International editions, but I couldn't quite see which one. Hurley's unsure about going through Sawyer's things, but Charlie reasons that Sawyer would want them to. Back on the drugs, I see. Charlie's next theory is a little more sound, though, that people need food, and medical supplies and -- as he unearths magazines beneath the books -- "shocking amounts of pornography."
Charlie doesn't get much time to flip through "Playpen" magazine, because Desmond strolls over (Hurley, since he's in Sawyer's tent and everything, nicknamifies Desmond to "Desmondo") and solemnly tells them that he needs them to come with him. Desmond asks if he found Eko. "Both of you," is all Desmond says, and he turns to go.
They follow him out into the jungle, where Locke and Sayid are waiting, and they get right to it, explaining that Eko's dead; they buried him yesterday. "The island killed him," says Locke. Charlie asks what Locke means, and he has to ask it again, because no one answers the first time. "You know what it means," says Locke simply. Desmond seems a little preoccupied as Locke starts talking about how people are on edge with "the doctor" gone and don't need to have to worry about what's out here in the jungle. Well, so much for John "No Secrets" Locke as leader, huh? Locke says, with a straight face, that people are going to look to Hurley and Charlie to see how they react to Eko's death, so when he tells people what happened, he needs Hurley and Charlie to help keep things calm. Hee! Hee hee! I mean, I can see Hurley being more or less liked by everybody, but does Locke really think everyone follows a What Would Charlie Do philosophy?
Desmond's starting to look a lot more agitated, and Hurley notices, but he's been a little more attuned than anyone else to Desmond's recent strange behaviour. "Hey, guys, what's wrong with Desmond?" says Hurley, and with that, Desmond is sprinting through the jungle, back to the beach, where he strips down his pants and starts swimming out to sea, through the rather choppy surf.
The rest of them, even Hurley, are close behind, wondering what the hell he's doing, until we (and they) see that there's someone else out there. Then Charlie notices Sun strolling up, holding Aaron. "Where's Claire?" he asks, and Sun says she went for a walk, because Sun offered to watch the baby. She's not even finished with that sentence when Charlie is off and running to the water too. Nice that he didn't give a shit until he realized it was Claire out there. Not that anyone else seems to be doing anything either.