Season 4, Episode 13
"Sometimes a Great Notion"
Well, that was pretty bleak..
That whole episode was an endless succession of bleak events: Earth was nuked; Earth was populated by Cylons; Roslin is despondent and burns the prophecies; Adama is despondent and tries to get Tigh to kill him; Kara finds her own body in the raptor and Leoben runs off in fear; Dualla kills herself; D'Anna is going to stay behind on Earth and die.
I did wonder why the "Previously on Battlestar Galactica" featured Dualla so much and originally thought perhaps she was toast in the episode. When she and Lee appeared to be picking up their relationship, I thought "well good, there's a bit of hope in a rather bleak series of events." Two minutes later, "Holy Frak!"
One of the most interesting aspects of this episode was that the character who, for my money, was the bleakest character of the whole series (Tigh) turned out to be the one who had the element of hope left in him. He refused to kill Adama, and he realized/figured_out that Ellen was the last Cylon.
So it seems that one of the threads of the rest of the series will be Tigh's attempt to find Ellen, somewhere. Also, we have to unravel the mystery of what happened to Earth and the human/cylon relationship in general. This will most certainly be tied up with the statement "what happened before, will happen again."
My thoughts: humans nuked the Cylon planet. A few survivors picked up the remnants and moved to one of the twelve colonies, where they started over and made humans in an attempt to better themselves (much as the humans made Cylons in their image). Or something like that. It doesn't quite yet explain why the human-like Cylons nuked the humans.
Summary: Overall a nice, if bleak, start to the rest of the series. I look forward to where things will go from here. There are mysteries to be resolved, and hopefully some sort of positive resolution at the end of it all. One can hope.