The "Unicorn In The Garden" Rule

This rule is intimately connected to the principle of Willing Suspension of Disbelief.

The rule is, quite simply:

"If it's required by your plot, make one fantastic assumption in your story and only one -- and do it in or before the first chapter (or first page or two, for shorter works). Do not add more as the story goes on. And once you have your one assumption, all further fantastic elements must derive from it, not any new assumptions. And just to be fair, it must be either obvious to the reader, or something that can be deduced from evidence present in your story."

(And yes, having the whole point of a story being the process by which the reader finds or figures out the divergence which changed the entire landscape is perfectly valid, although if badly handled it can become little more than a Tomato Surprise.)

The name comes from the example of James Thurber's classic short story "The Unicorn In The Garden". Put simply, Thurber's story is about an ordinary suburban couple who wake up one morning to find that there is a unicorn in their garden. The story works because the only fantastic element is the unicorn. If on the second page a flying saucer had landed in the garden next to the unicorn, it would not have been as strong or as good a story.

This cannot be emphasized strongly enough to the writer, beginning or experienced: One and only one "unicorn" should be in play in a story. If you have two or more, you have a case where you need to find a more general "fantastic assumption" that allows for all -- or you have several different stories demanding to be written and colliding inside your head.