Your novel's premise is a description of the plot in 1-3 sentences. Think of it as the synopsis potential readers will see on the back of your book (although the actual synopsis published will be a bit longer).
It's good to create one because it tests your own understanding of your novel's plot. But make sure to cover the premise in plenty of mystery--it's meant to interest possible readers, and nothing interests a person like mystery!
The main character, setting, and inciting incident (or main conflict) should all be mentioned.
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