Quantum Timeline

Is the multiverse real, or is there only one space-time continuum? Can there be more than one Once Upon a Time? Is it possible to travel between the potential timelines? If so, can someone change the past or alter a timeline? And what does that mean to the generations who should have come after that alteration, but no longer do? Bill Engle is on administrative leave from his position as a detective in the Capitol Heights Police Department. Then the terrorists bring down Air Force One with Stinger missiles. The emergency puts him back at work, but also leads to Martial Law and Engle's involvement in an Ultra Secret project. Time Travel might be able to reverse the assassination of the President of the United States. If so, why not reverse previous historical calamities: Other assassinations? The rise of Stalin or Hitler? The Great Chicago fire? 9/11? Engle learns that tools can become weapons, and changing history may be more dangerous than not interfering with the space-time continuum.