You really have to watch and pay attention closely to understand this story to the full. Although the first few episodes weren't captivating, afterwards you'll understand more about the ability of contractors. Only towards the middle/end will you learn about the protagonist and his background and how it will effect the future. I'm not the biggest fan of slow beginnings, but the more you watch the anime, the more you'll understand, and that will give you a reason to binge watch in one night;)