Much of Season 4's charm has been the structure - the writers are clearly aware that the flashbacks in every episode had become laboured and uninspiring, and in Season 4 they've done their best to subvert the formula at every opportunity. This week, everyone's favourite former monk, Desmond, gets the flash-forward treatment - but not in a way you'd expect.

Last season, Desmond laid some hints about the island's recently-confirmed time-traversing properties when, rather than merely flashing-back, he actually travelled into his own past and found himself able to influence it. This episode sees Desmond's past self flashing-forward into the present, and being naturally very surprised to find himself on a boat full of people he hasn't yet met, thousands of miles away from home.

What follows is an incredibly tense episode as the Desmond of the past fights for his own sanity - and eventually life - to try and resolve what's happening to him. Among his tasks are convincing his spurned lover Penny that he's not crazy, and contacting Daniel, the physicist from the rescue boat, at Oxford University in 1996 to try and help his time travel experiments. It might sound a tad confusing, but in execution it's brilliantly simple and as compelling an episode as any television programme, let alone Lost itself, can get.

In order to save himself, Desmond has to find an "anchor" that exists in both time periods to help him psychologically reboot his brain, ending the time-jumps that will kill him - and he chooses his only love, Penny. It's a fairly mawkish premise, but Henry Cusik (Desmond) instils a perfect mixture of faith and desperation into the role that such openly sentimental pseudoscience is instantly forgiven. The episode's penultimate scene, in addition to containing a fairly major plot development, also contains one of Lost's most emotionally intense moments, getting even your stony-hearted reviewer choked up.

Utterly magnificent. Somebody get this episode a Hugo Award or something.

Desmond's plot naturally intersects with a few other scenes - He, Sayid and the Lawnmower Man reach the almost-mythical boat, only to find themselves shut out of almost all conversations and few answers forthcoming. The sets have a run down, eastern bloc sensibility that suggests that this boat, however useful, isn't going to be much of a ticket out of there.

Meanwhile, the aforementioned Daniel Faraday gets plenty of fleshing out, revealing himself as an apparently well-meaning, if damaged character with his memory crippled by his own time-travel experiments. There's legitimate concern that introducing time-travel into Lost opens a very difficult can of worms, and it's all too easy to look at Alias, JJ Abrams’s' previous show, and see how the quality declined as the science fiction become more evident. Is Lost strong enough to deliver such concepts without losing either itself or its audience? If the strength of today's episode is anything to go by, there's nothing to worry about at all.