- Be observed by the Players' Characters;
- Be able to be influenced by the PC's;
- Be of Interest to the PC's.
A plot that the PC's cannot or will not influence isn't a plot it is a box text railroad or short story entertains the GM and maybe, just maybe the players.
A plot that doesn't interest the PC's will be ignored and thus it will not be influenced or observed by the players. Again, it becomes entertainment for the GM at the expense of the Player's boredom.