To begin, this book definitely feels like what it is--the middle book of a trilogy. Largely, I felt that it served as the link between the first book and the third. I still enjoyed the book, but I didn't feel that it had the same magic as Han's debut book.
The book takes place during the summer after The Summer I Turned Pretty, although it includes plenty of flashbacks to the intervening year and even before. After dating briefly, Conrad and Belly have broken up. Even more devastatingly, Susannah has died. Now it's Belly's first summer ever without a trip to the summer house. She's stuck at home and isn't enjoying herself, in spite of her friend Taylor's best efforts. But then Jeremiah calls, saying that Conrad has disappeared from school. Jeremiah and Belly set out to find Conrad, and Belly finds herself caught between them.
The entire story takes place over the course of about three days, but as I said, it does include flashbacks. One complaint that I had with this book is that Belly's current immature attitude and the flashback episodes don't make her relationship with Conrad seem very serious. While I understand that she had loved him for her whole life, this book left me with the impression that their dating was casual at best. So I understood that she was having a hard time getting over him, but I didn't really see why she expected him to think it was such a big deal. More flashbacks explained this much better in the third book, We'll Always Have Summer.
My other complaint is that in this book, Jeremiah is painted as pretty much perfect. He's a loyal friend in the first book, but here he seems to have no faults whatsoever. Clearly the author is trying to establish him as a better choice for Belly than his brother--which, for the record, I do think he is. But he was just portrayed as so all-around perfect that I had a hard time believing in him as a character.
Overall, a quick read and a nice story, but definitely the weak link of this trilogy.