Desmond's ability to see the future surfaces again as he somehow realizes Claire is in danger from drowning in the ocean and rushes off to save her. Hurley and Charlie are suspicious but amazed and get Desmond drunk, hoping that he'll open up and tell them more.
We see Desmond having what may or may not be a flashback after the Hatch explosion. He's certain that his visions aren't just his life "flashing before his eyes," but that he's actually been transported back into time, back to London, where he's planning to ask his girlfriend Penelope to marry him, before he goes on the round-the-world sailing trip that landed him on the island.
It's like he's trapped in a time loop and he's convinced, if he changes things, he can end up with Penelope and not on the island. He sees Charlie in the flashback/forward, but Charlie thinks he's crazy, like they've never met. And maybe they haven't...yet. When trying to buy an engagement ring for Penelope, a woman in a jewelry shop tells Desmond he can't go through with the purchase, he's supposed to have second thoughts, so he'll end up on the Island, pushing the button. We don't learn who this woman is or how she knows what she does, or even if she's real, not a figment of Desmond's imagination. The flashback ends with Desmond being knocked unconcious and then awaking after the Hatch explosion.
He also tells Charlie that he wasn't just saving Claire from drowning. In his vision of the future, Desmond saw Charlie drowning trying to save Claire. So Desmond went instead. Earlier in the season, when Desmond erected a golf club lightning rod to save Claire's shelter from getting zapped, it was also because he had a vision of Charlie dying. Desmond gives Charlie the warning that, ultimately, fate will catch up with him. Charlie's days seem numbered.
Clues, observations, speculation:
* The episode touches on the same "what's fate, what's in my control, what if I'd done this instead" themes present throughout the series. Was it the various choices made by the characters that has resulted in them being on the island? Was it fate or happinstance?
* Busking on the London streets, Charlie is singing "Wonderwall" by Oasis, which contains the line "maybe you're going to be the one who saves me."
* The sign on Charlie's guitar case identifies him as "Charlie Hieronymus Pace." "Hieronymus" means "holy name." Also, Hieronymus Bosch was a Dutch painter whose works depicted sin and human moral failings. More.
* Maybe more interestingly, this site details the life and work of a researcher/engineer named T.G. Hieronymus who "studied plants and minerals and their relationship with different forms of energy, including the emanation of energy from these materials" in the 1930s and developed theories about eloptic energy: "Eloptic energy radiates from or is is in some manner given off from, or forms a force field around, everything in our material world under normal conditions at ordinary room temperature and with out any treatment of any kind. The energy radiated from each element differs from every other element in frequency, thus we have a means to determine the contents of a material without disturbing the material or having to excite it in any way...Hieronymus concluded that, Eloptic energy can be used to analyze any material, even from a photograph, and it can be used to alter the material, treat it, disintegrate it. Plants , animals, humans, chemicals, drugs, everything...." "Eloptic" is a contraction of the words "electromagnetic" and "optical". Google around a little and you'll see references to Hieronymus' work and ESP, alternative reality, UFOs and more, plus lots of references to the Hieronymus Machine. Seems like the kinda wacky, weird stuff that might inspire the writers of "Lost."
* Desmond's last name is "Hume." The philosopher David Hume believed "all human knowledge comes to us through our senses." More.
* A painting in Widmore's office shows a polar bear, an upside down Buddha (an image remiscent of Ben hanging upside down in one of Rosseau's traps) and the Buddhist greeting "namaste" (used in the Dharma orientation films) spelled backwards. Namaste means The painting is similar to those seen in the Hatch.
* The digital clock in Desmond's home reads "1:08," reminding of us of the counter inside the Hatch.
* A delivery man in Widmore's office building mentions an "8:15" delivery. The plane that crashed on the island was Oceanic 815.
* The microwave buzzer in Desmond's place sounds like the computer alarm in the Hatch.
* In the pub, the jukebox twice plays "Make Your Own Kind of Music," by Mama Cass, a tune we first hear Desmond spin on the turntable in the Hatch.
* Widmore is a mean bastard.
* Is the lady in the jewelry shop the same as this woman who appears in the teaser for next week's show? I don't think so.
* The red tennis shoes sticking out from a pile of rubble seems like a joking reference to the "Wizard of Oz."
* "Admiral MacCutcheon," founder of Widmore's favorite whisky, is a character in Jules Verne's "20,000 Leagues Under the Sea."
* The lady in the jewelry shop's last name is "Hawking," perhaps another reference to Stephen Hawking.
* There are ads for Oceanic Air and Apollo candy bars on the soccer field as Desmond watches the game on the pub TV.