Oh my. This is one of the best follow ups I think I've ever read. 
 It's gripping, edge of your seat the whole time, eyes partially covered, feel all the feelings kind of book. 
 In a very dark and yet very good way. 
 There's almost no way for me to describe the plot without spoilers so just know that if you liked Bang, you will LOVE Echo and I am seriously DESPERATE for the next one! 
 Gah!