Your book needs a theme. A theme is closely aligned to your plot and often the words are used interchangably. I see them as different however. A theme is more of a feeling your book has, a message, or a arc your character will undergo. Your plot is the step by step way in which you are going to tell story. Of course they are closely aligned. But building a plot takes a more practical method of working out how your character is going to move from A to B.
The list below is from 20 Master Plots & How to Build Them (Ronald Tobias 1993). See if you can identify your plot in one of these. It may give you focus. It may give clarity to a story goal you are battling to lock down.