The premise is simple. What is the most logical thing for the first time traveller to do? He would travel to the future to get his hands on the most advanced time machine possible. In the end, the best is the human mind. They time (and space) travel with a simple thought.
It has a single "timeline", i.e. changing something in the past doesn't "split the timeline" but creates a paradox, that translates to "frag" on all people related to the changed action. This makes people want to correct these paradoxes.
Example, I go back in time and kill Hitler. It frags a LOT of people. The good guys want to correct the frag. They could go back in my timeline and prevent me from doing it, fragging me. If they don't want to frag me, they could switch Hitler with a Robot or something. I don't get fragged because still I shot something I though was Hitler, yet Hitler remains alive.