What is the main plot in the novel, Keep in Touch: Letters, Notes, and More from the Sisterhood of the Traveling Pants?

The book has no plot of its own, but rather is a collection of notes, letters, and other items that the original book's four major characters have written to each other during the course of their lifelong friendship. Much of what happens in Keep in Touch is vague and somewhat confusing because the author assumes that the reader is familiar with the original book.