To repeat what's already been said, this really was a fantastic episode! So so good...'Flashes' was a great episode but confused me greatly, but I was pleasantly surpised to find that I could keep up with this one!
The sci-fi element was done in such a good way, but all the technology or heavy weight concepts didn't push this episode into a great one - what did that was the emotional core of it, with Des being so confused and scared but still determined to hold onto his anchor Penny...aw! Des/Penny is the only decent love story on the show at the moment, and puts Jate and Skate to horrible, horrible shame in my view...I just find it impossible to care about the two 'lead' romances one single bit when you compare it to just one scene of Des and Penny. I'm so happy Penny actually picked up the phone! lol
And Daniel, bless you! He's such a good guy, and I love him as a random Oxford Professor...but he does need to cut his hair a bit back then though...and he's got Des as his constant!
Only downsides to this episode is that Jack, in his short amount of screentime, still managed to be an annoying pain by whining on about not getting anywhere with the phone (okay I get why he would do that but still...) and then for trying to interrupt Daniel when he's trying to help Desmond. Think you can do more for him, huh Jack? And Charlotte was just irritating again.
And I could almost forgive the lack of Locke and Ben since this episode was so great. Almost
Such a relief from the dissapointment of Eggtown when it came to characterisation, since The Constant actually made me care - care a lot - as to what happened to Des, Penny, Daniel and, even in his short space of time with us, the ferris wheel riding Minkowski.
Definately the best episode of S4 yet, and definately up there in my top 5.