This episode, in a nutshell, sums up Lost and why I love it. It's as interesting, exciting, dramatic and as damn cool as you could ever ask a television show to be.
Using a similar plot device to the series Finale of 'Star Trek : The Next Generation', this episode deals with a fantastic plot-device crafted by the writers known within the fiction of the show as 'Consciousness Time-Travel'. The character of Desmond finds himself 'unstuck in time', essentially meaning his consciousness is sent hurtling between the present and the future versions of himself. This strange phenomenon, seemingly brought on by the mysterious time-distortion properties of the Island, makes for some excellent storytelling relating to Desmond's character. As well as explaining the origins of his premonitions last season, the time travel element within this episode helps us further explore the greatest romantic relationship of the series; his love for Penelope, a past girlfriend whom he had split up with during his prior time-traveling romp, last seasons mind-blowing 'Flashes Before Your Eyes'.
The Constant is remarkable in that it somehow manages to tell an excellent stand-alone story, filled with plot, drama and emotion, whilst also bursting at the seams with dense exposition, character development and an excellent narrative. This neat little package is compliment by a significant step in the over-all, unfolding story-arc for the series, and honestly each time i watch 'The Constant' I am simply amazed that the writers managed to fit all of this into one single, 42 minute episode.
A fine example of why I can't get enough of Lost, and easily one of my favorite episodes of the series so far.