And again, based on last week's game of Next Chapter Please - you will need a book you've never read, by an author you've barely heard of. 

Open the book to somewhere in the first third-to-half of the book, and find the next chapter or section break. Read that chapter or section break (if it's a really short or really long chapter or section break, read 20 - 30 pages.)

Put the book down. You may want to jot down the names and events you just read, briefly. Put the book away. No looking at it anymore.

Now, instead of writing what happens next, you have to write what came before. How did they get here? What's really going on? Why is there a banana in the wardrobe?

You may find it helpful to Work Backwards, if the story's particularly complex or you can't think of a good beginning.