Time Travel can cause a lot of damage. You could run into the Butterfly of Doom, leave behind a Timeline-Altering MacGuffin, try and bump off Hitler or create a Temporal Paradox. Therefore, in any society where Time Machines are easily accessible, deliberately altering time will be a crime. In addition, there may be people, most likely a police force or The Men In Black, who fight those who [...] [tvtropes.org]