I don't really understand the inFamous storyline?

I'm currently on the second island and I don't really get the story. It was all simple until we had to fight some chick named Sasha. I could understand she was the boss of the reapers, but then whats this plague they speak of? and then a couple of missions later, some guy named Kessler calls me, and then some guy named Alden kidnaps Zeke. What is going on? Do the Dead Drops explain the story at all? and NO SPOILERS PLEASE!