So I have this idea that I really want to do a story about, screenplay, novel, whatever. It's about an "airport through time" called a time-port. The currency exchange desk will change 2008 dollars for 1970 dollars instead of one currency for another, security will check you for anachronisms instead of weapons (no iPods before the 2001, no Chuck Taylors before 1917, etc.), but functionally speaking, it's just like going to the airport. You go to your 'gate' and get sent back in time, then for your return trip you go and stand where your gate is going to be. Naturally, interfering with history in any way is verboten. In fact, instead of having TSA workers and air marshals, the time-port has people whose job is to preserve history (of course, if they failed they'd never know, would they). I figure the time-port is situated just outside Los Angeles, so the Wild West, the Golden Age of Hollywood and the Watts Riots are all possible destinations for time tourists. My problem is, I can't come up with a good overarching plot to go with the whole 'time-port' concept. I have a lot of smaller ideas: a man runs through a crowded room, then has to run through it again and again, until he realizes the whole crowd is him; a time police man is bitter and disillusioned because he had to save Hitler from a time traveling assassin; a common cold from 2525 becomes an incurable epidemic in the present day; a boy takes a girl on a date to an old fashioned movie premiere; a climatic showdown during the meteor shower that killed the dinosaurs, or while the sun is going supernova. But I don't have a big overall story, and I'm struggling to come up with one. I know I want my major theme to be "you can't change the past, you have to learn from it" but other than that I'm stumped. Help me out here?