In my experience as a writer, a synopsis has two purposes. Its first, official purpose is to summarize the plot of the novel you are about to write. Its second, far more important purpose is to show whoever might read it that you can, in fact, write – and to get them hooked on your story, your characters or both.
The key word here is ‘show.’ Rule one for a good synopsis: Show, don’t tell. So if you are planning to write a funny book, then your synopsis needs to be funny. It is no good telling an editor or an agent “I am going to write a funny book.” The synopsis is your chance to prove that you can actually pull that off. Because let’s face it, if you can’t be funny in your synopsis, you are unlikely to be able to sustain a side-splitting novel for a hundred thousand words. Similarly, a terrifying thriller needs a terrifying synopsis, a poignant romance needs a tear-jerking synopsis, and so forth. (And yes, I do realize this is deeply obvious advice, but you would be amazed how many aspiring writers don’t follow it.)
The second rule of a good synopsis is: Keep it brief. The ‘One in Four’ new writer competition is asking writers to submit a 500 word synopsis. You are giving a would-be agent or editor a tiny, tantalizing taste of your writing, characters and story – enough to make them want more, enough to have them coming back to you with questions. Never give anyone everything they want at the very beginning! (That’s a life rule, but it works for synopses too.) On a more mundane level, a ten thousand word synopsis will go straight in the bin. It doesn’t matter how good it is if it isn’t read. Nobody has time.