Right off the bat, I would like to say it was a refreshing change to read a book with main characters in their 50s that shows that older people are *not* asexual and/or boring.
The action of "The Last Time I Saw You" is centered around five people attending their 40th class reunion. Candy was the class queen, Pete was the Big Man on Campus, Mary Alice was the plain Jane studious type, Lester was the nerd who became a veterinarian, and Dorothy was the one who had only a couple of close friends and kept others at arm's length.
Each of them has seen life go in unexpected ways since leaving high school, and it is those stories that come through as each character expresses concerns about attending what has been announced as the last class reunion.
As one who avoided class reunions studiously, I admit that it was interesting to see what went through each character's mind. I imagine that this is very much true-to-life; Mary Alice's concerns rang very true for me as reasons that I have actually stayed away.
To say much about the reasons and life issues for each character would create enormous spoilers. The book was entertaining and well-written, populated with characters who felt real, and very much enjoyable.