Nick and Kate are married and run an ill-defined security business that involves running around in leather and shooting and/or beating up bad guys. They have an odd sort of chemistry - Nick is a Chicago wise-guy and very tough, Kate's motivation for doing this is unclear (although the money is good), they love each other in spite of their differences.
The central issue between them is the Game. Nick doesn't see why Kate has to play the Game at all, of if she must, why she has to follow the Rules. In fairness, the whole Highlander franchise suffers from this basic unresolved problem. Anyway, the question is whether their star-crossed love can ever work, and how many bad guys are going to die before we find out.
Since this is a series pilot, the central question is never resolved. Some standard-issue bad guys get whacked, some good tough-guy tough-girl banter gets exchanged, and we are left thinking that this spinoff had at least as good a chance as the one they actually did, which failed after one season.