Pathfinder

 Pathfinder has its own special brand of logic.  In Pathfinder, things that do not happen influence things that do happen.  Umbo went back and warned Rigg.  Rigg changed his actions.  Umbo then decided that he actually now didn't need to go back in time, because something that had happened when the warning wasn't in place now didn't happen.  It was a bit confusing, but I got the feeling that it broke logic up a bit.  Besides that, it was very well written, with an original storyline and several original elements in said storyline.