Traveling into the future is easy. To a small extent anybody who has ever flown has traveled forward in time (faster than the rest of the world travels through time). Traveling backwards through time however was impossible until the invention of the time machine.
Jared Mills and his friend Aaron Collins have been trying to discover what would happen if the past could be changed to create a better future. For example, what would happen if Hitler was killed as a child? In Hitler's youth it certainly would be easy to get to him. Unfortunately, Jared and Aaron discover it is impossible to change a past you have been changed or created by. Because Jared and Aaron would not have been born without Hitler they cannot kill Hitler.
To circumvent this problem they create a secret underground colony in Switzerland. The colony allows them to no longer be affected by events and people of the future that they would want to stop. As the colonists (or Time-Keepers) quickly discover it's still not that easy to change the past.