what is the synopsis of the story?

There are a few themes in this story. A larger one is "time". Time is not linear in the story. Instead it is broken up into certain moments. It's very much like how we remember the past. Certain select moments make up the bulk of our memories. In one part of the novel, ten years is compressed into a few pages. It reflects how the mind deals with not only time but the emotions that go along with it. You can check out other themes at the link below,

http://www.gradesaver.com/to-the-lighthouse/study-guide/major-themes/