A plot is basically the story, or the skeleton upon which you “hang” the other decorative parts of the tale (your descriptions of the setting, characters, and other little details).
So, how many skeletons do you have hanging up in your closet? Do you write down every single plot idea you’ve ever had, even if you’re in the shower and there’s no clean towels, or you’re dreaming the idea at 2.30A.M.? It’s a good habit to do so if you scheme like a villain, ( i.e. if you like to have a plan A and a plan B all the way to plan Z.) That way, when you need an idea for a story, you can flick back through your notebook and find a plot idea you haven’t tried yet, or cherrypick elements from more than one, mixing them together like a mad scientist in a lab. (Laughing maniacally is optional.)
Or, perhaps you plot methodically, one step at a time, like a gardener planting rows of beans and tomatoes in their allotment. As soon as you get the idea for a story, or your seeds, you start writing it and get the words down on the paper until you reach the end. A story can morph and reinvent itself in surprising ways using this method.
Both of these ways of writing are absolutely great. There is no right or wrong way here (so long as you’re not plagiarising someone else’s ideas, as that is so not cool). You do you.
Editors are interested in reading stories that have unusual plots they have never encountered before (or at least, not encountered often). The problem with this is that, according to Robert Heinlein, there are three basic plots that absolutely every story ever told (or is told, or will be told) is based on:
1. Boy Meets Girl (or girl meets boy, or boy meets girl too late, or fails to, and any other iteration of this scenario)
2. The Little Tailor (or rags to riches)
3. The Person Who Learned Better (someone whose views or beliefs change as a result of events).
But wait… someone else said there are only two plot types:
1. Someone goes on a journey
2. A stranger comes to town.
And someone else (Jorge Louis Borges) said there were actually FOUR plot types:
1. The siege of the city
2. The return home
3. The quest
4. The sacrifice of a god.
There are other writers and analysts who believe there are seven plot types, or five, or even thirty-six. The point is, whether a plot idea is new and interesting depends on the person reading it. It is a matter of subjective opinion.
Therefore, whatever your plot idea is, if you read a lot of books and your plot is unique and interesting to you, there’s a chance that an editor out there will think the same of it as well. It’s just a matter of finding the right editor, so send your stories to as many places as you can (that are a good fit for the story, of course — and as long as you mind the guidelines concerning simultaneous submissions.)