Well darker than black is basicly a story (anime show) about contracters that give up their life of being mortal by reciving a special power or ability....but with a catch.....they also pledge their soul to the darkness.....yet like Hei.....there not all bad....they want to escape the mistake they've made and escape all those other ruthless killers that lurk around the darkness in a battle for power and the fear of others.
In about 23 episodes you'll gets a short explaination about contracters..their life...HEI.....about what exactly is a contract.....but the show unravels it slowly and it's one huge puzzle.....the viewers job is to put the pieces together.
So thats briefly...in vague terms.....Darker Than Black.......yep!
X3