Like I said yesterday, a one page summary divulges the entire plot of your novel. Another word for it would be a plot summary. So, to help people understand what I mean by plot summaries, I went and found a few good ones on the web.
The first one is the plot summary for Madeleine L'Engle's A Swiftly Tilting Planet. You can find it on wikipedia here. Just look at the part of the page labeled "Plot Summary," not all the other things on the page.
This summary does a good job of detailing out every major plot point in an interesting manner. Granted, it contains lots of spoilers, but that's exactly what an agent or editor wants in a plot summary. They want to see the entire plot arc.
Now because this book is one in a series, the plot summary does not have to introduce the main characters. It's assumed that the reader, in this case random wikipedia readers, are already familiar with the characters in question. If you are pitching another book in a series to an editor who is familiar with your characters, then you can also be as brief about your characters. Otherwise, you would want to tell a sentence about each major character.
Another great source of plot summaries are any of the study guides designed for students. As an example, I've linked to SparkNotes plot summary for Treasure Island. You can see the page here.
These plot summaries are designed for students who do not have time (or desire) to read the assigned books. In a short summary, SparkNotes, Cliff Notes, and the like tell the reader about key characters and all major plot and subplot points. These are the same things an editor or an agent needs to see when they are trying to determine if they would like to read a certain work.
So, keep practicing on those summaries. Remember, the best will have their manuscripts requested.