Ah, the query letter. How tempted we are to jam the entire book into a query letter. How can we possibly resist? YOU MUST RESIST! The query letter is meant to give the editor/agent a brief overview of your book and what makes it different than the hundreds of others they have seen with the same plot. Consider it your one-minute elevator pitch. If you are telling too much in the query, you have not practiced your pitch. Impress the editor/agent with your brevity and ability to write succinctly. If you query letter just won't stop telling, they will assume that your manuscript just won't stop telling either. Check out this article from Query Shark for a great example.