This series is a wonder of modern science-fiction. The script by Jeff Lemire puts unique spins on several classic tropes and every issue so far has found at least one way to surprise me. And the artwork by Dustin Nguyen is brilliant in its subtlety.
If you haven't given Descender a chance yet, this is the perfect jumping-on point. This issue is the first part of the second story arc - Machine Moon. And a handy blurb at the front of the book tells you everything you need to know. I'd still suggest tracking down the first six issues or picking up the TP collection, though.