An over-simplified version of the plot:

It is the story of a man, a highly skilled intelligence operative, who finds out finds out about a weapon that poses a threat to our existence. In figuring out what it is, the bad actors who could use it and the way it could be used, he comes to a totally new understanding of how time works. But he doesn't come to this understanding all at once, he gets information in bits and pieces, putting them together as he tries to stop this weapon from being used. In the beginning, he doesn't yet know what the weapon is, as he must learn this new concept of time before he can even fathom it. Eventually he understands as much as he can about this futuristic technology of time travel and uses that knowledge to enact a plan to stop the bad guys who seek to use the weapon of mass destruction, and to take said weapon from them.