One of the most common questions I get from aspiring authors pertains to protecting their work from plagiarism. I can't speak for screenwriting, but in the publishing world it's a misplaced worry.
As a writer, you simply have to take a leap of faith that no one wants to steal your work. It just doesn't happen. Agents and editors are not sitting at their desks, rubbing their hands together as they await their next victim. Nor are other writers trolling the Internet looking for ideas to steal. I mean sure, you can probably find a few examples to prove me wrong. But you can also find examples of people choking to death on water. The point is that the odds are so astronomically low that they don't merit concern.
The sad truth behind this is that it is so difficult to make any money from writing that there's no incentive for theft. We all believe in our work, and cling to the hope that we have the next million dollar idea. But realistically, why would someone risk plagiarizing, only to have to spend a year researching and queryng agents, rewriting the letter again and again as it gets rejected? And that's not even considering how many months of rewrites will follow before there's even a slim chance at publication and an even slimmer chance of making any real money at it.
And one final point. As the author, you automatically own the copyright to your work and have legal protection. So try to relax and not let these fears get in your way.
Instead, worry about the real things ... Are you creating believable characters your readers will care about? Are your storylines fresh and exciting? Are your sentences exquisite and evocative? Is there enough coffee, vodka, or red wine in the house to get you through the week? (Water, as you know, can kill you.)
Now get back to work ...