I would love to give you a nice overview of Darker Than Black, and be able to talk to some extent about the wonderful intricacies of the plot. But that would require me to first figure out what the plot is going on about, which is way more complicated than it sounds.
I get the feeling that at some point the writer played through Dark Souls, and immidiately leaped out of their seat, yelling about how they have found the key to their next plot. So nearly all of the world-building of Darker Than Black is done in the very Souls-like style of not really explaining anything, instead letting the inter-character dialogue build the world around them. A style which, although confusing as all hell at first, works really well once everything is established.
The Cliffnotes of Darker Than Black is that some weird wibbly wobbly magic stuff happened, and a few parts of the world became really overcharged in magical energy, or something of the like, and are now really dangerous. This event also gave tons of people "magical" powers, for whatever reason, that although useful, extoll some price from the user whenever activated. These people became known as Contractors, and the show follows one particular group of Contractors while they...well, I'm sure they accomplish something...
To be honest, the bigger overarcing plot isn't really what is important, so much as the moment to moment plot. One character wants a thing, another character doesn't want to give thing up, and awesome action ensues. And I mean it when I saw awesome. The animation for this show is supreme, the music finds the perfect middle ground between pumping, but not overpowering, and the characters themselves have this style about them that makes watching them butt heads supremely entertaining. And that is all before mentioning that this is one of the few shows I have seen in ages that is fully consistent with the rules of the powers the characters use.
So while maybe not the peak of well articulated world-building, Darker Than Black is certainly the kind of excellent action.