I am really glad that I read these in order, but it's not a sequel. I've heard it referred to as a "sidequel" instead. Where Oryx & Crake ended, the plot only advances a tiny bit at the end of this one. The events are concurrent, and offer a different perspective on the waterless flood/plague. Atwood has created a very rich world here. I felt it wasn't as horrific as the first, but that may be because the events are the same so you don't get the same shock value. I am eagerly awaiting the third book in this series!