All writing needs to contain threads of believability.
Imagination is a handy tool to utilize when creating a piece of fiction. For some writers, the products of imagination come bubbling forth as a gushing stream, a freshet tumbling its path down a rock-strewn hillside. One situation sparks another, even more stupendous and climatic than the previous. But there is a misunderstanding that a writer can put anything down on paper, make it interesting enough, startling enough, shocking enough and it will be a success.
But through all the astounding and mind-plowing writing there must run a thread of believability. Anything imagined can be integrated within a story's structure as long as the writer remembers to embed an element of plausibility, giving the reader a hand-hold to grasp as the words take him or her into uncharted territory. It's all right if your writing causes your readers to expand their understanding of what is real and normal. But unless your writing contains a nugget of believability, the story will be rejected and discarded faster than a red-hot coal.
It isn't required that the writer never strays from the norm that we all encounter in our everyday lives. A piece of fiction should take its readers into unfamiliar territory, introduce them to unusual circumstances, present people who think differently and live uncommon lifestyles. But through it all the writer must keep in mind the commonness of mankind; should include those basic elements and truths that are a part of everyman's psyche.
To enable your readers to accept the reality you have created, those concepts and ideals that are an integral part of being a human being must be evident: wants and needs, love and fear, disappointment and success, to name just a few.
These will become an anchor for the readers, allowing them to continue with you into this unfamiliar world you are creating on the page. As these benchmarks impress themselves on the readers' minds, they will allow the reader to feel a sense of comfort because in the midst of their adventuring, the reader will remain in contact with the believable.