I've noticed how often the same story line is used by authors on these forums, including me, whether consciously or not. Sometimes it's what you could call convergent evolution - not plagiarism. As you say, there aren't that many plots to play with. But all artists and writers pick up other people's ideas and develop them in their own way. In music it's called 'variations on a theme' - isn't that what we're all doing, being creative with themes?

Plagiarism is copying and passing something off as your own without acknowledgment, which isn't the same thing. I can't say I've seen any evidence of that, though I've heard people saying it's happened to them and it must be extremely annoying.