It's about a woman who is basically just floating through life, until her husband decides that he's unhappy and leaves her. She ends up in a pretty deep depression, her boss makes her take a sabbatical, and she ends up moving in with her parents for a while, at the age of 38.
This coincides with her upcoming 20 yr high school reunion. She basically reverts back to her high school self, reminiscing about how it was the best time of her life. It wasn't until later that she discovers/remembers just how it really was for her, and this basically makes her "grow up."
It's a pretty good book. A quick read. I checked it out on a Saturday, and it was finished by Sunday night, reading sporadically. It's a good nostalgia story for anyone who grew up in the 80's.