I am sure you have all had this experience before. You are reading a story and stop when you start seeing a similar story plot element from a book you previously read. What??? Did this author plagiarize the other author? Probably not.
Let me first say that plagiarism and copyright infringement is indeed a serious issue. In our digital age and the availability to access information quickly, this can certainly become an issue. However, it is important to understand EXACTLY what plagiarism is before calling an author out on this.
I am going to one of my favorite sources for all things writing: A WRITER'S REFERENCE by Hacker & Sommers for this. This definition is pretty easy to understand.
"In general, these three acts are considered plagiarism: (1) failing to cite quotations and borrowed ideas; (2) failing to enclose borrowed language in quotation marks; and (3) failing to put summaries and paraphrases in your own words." (Hacker & Sommers, 2011).
While this might sound entirely academic, it is the one word I want to focus in on - "BORROWED". What this is dealing with IS NOT plot elements. A story with a missing baby or a father who might have secured their son a position with an organization is something that happens all of the time. Borrowed also does not mean similar things that happen chronologically in a story. Again, these are pretty common. In the academic world, this would be similar to the idea of "common knowledge." These are things that do not need to be cited because most people should know this information.
I am going to stay in the academic world to explain this better. When we talk about borrowed ideas, it would be things such as talking about Darwin's theory of evolution and claiming it as your own. For publishing and certainly the fiction world, having a same name of a character IS NOT plagiarism. Names are common.
In the writing community, the issue of plagiarism really comes down to the idea in item number 2 of "borrowed language." Copying a phrase, or phrases or paragraphs IS plagiarism. I cannot simply go out and find a great paragraph an author wrote and claim it as my own. Now, be careful. You cannot push this to single sentences, especially if it is considered common language. For example, "When Michael touched Jane's hand, she felt an intense spark of sensual energy." Look, I made this up, but my bet is you can find something pretty dang close to this in someone else's writing. Not plagiarism, but a common nuance of romance writing.
So, be careful out there. Just because someone has a similar story idea does not mean they stole it from you or plagiarized.