bendover:
Ideas, names, titles, plots are not copyrighted. You're fine. An author has to show exactly where you wrote word for word in the story.
Actually that isn't quite correct. Ideas and titles are not subject to copyright, yes. Character names can in some cases be trademarked, but this is limited to major commercial characters as a rule. A really unique character name might be subject to copyright protection, but this would be very unusual. Plots, however, are a different matter. A generic plot, or a general plot outline, something like "Boy meets girl" or "Top spanks brat" is common property, and anyone is free to use it. However a detailed and original plot can be protected by copyright, and a work imitating it in detail, not just in general outline, but in point-by-point detail, can be found to be a copyright infringement. There have been cases so holding. But it has to be a similarity so great that no one could plausibly have invented the plot independently -- a similarity of basic concept is fine, and happens all the time.
I understand that mobile_carrot was mostly joking above about the similarity, but the idea bendover stated is incorrect. If one were to take an existing story and paraphrase it sentence by sentence, or even paragraph by paragraph, so that no words are copied verbatim but the new work is clearly a direct copy of the old, it would be a copyright violation. The more original and unique the plot, the more strongly this would be true. (Note that a translation into a different language without permission is also a copyright violation, even though no words may remain unchanged.)
As a practical matter no one is likely to sue over a spanking story, even in a clear-cut case of cut&paste plagiarism, and accidentally similar plots are not going to expose one to a charge, even with works commercially published. So bendover is correct in advising "don't worry", but his reasons are incorrect.