S4: The Constant

So what is happening to Desmond?  Is his flip-flopping back and forth between a certain date in the past and the present.  When he returns to the presents, he has no recollection of anyone.  His mind must still be in the past.  But then he's able to retain information from the present to take to the past and then from the past to the present.  It's semi explained that since he was exposed to radiation (the Hatch imploding) that when he left the island his mind starting transporting through time and if he didn't find a constant (something familiar in both the past and the present instances) then he would surely die.  

Well thankfully with this knowledge, he is able to go to the past and find Penny and tell her not to change her number so he can call her in the future.  When he calls her in the future, they have a heart warming scene, which I thought was definitely more moving the first time I watched it.  Then Desmond was fine and remembered he people he forgot in the present.  

The surprise at the end of the episode was in Farraday's notebook - "If anything goes wrong, Desmond is my constant."

Overall, this was a fantastic episode when I first watched it.  Unfortunately, a lot of the mystery had been taken away and it wasn't as emotionally moving the second time but the story was really well written and the bonus of seeing Mr. Widmore purchasing a book from the Black Rock was pretty cool as well.