There really isn't much of a synopsis necessary for this one because it's pretty much just more of the same story and characters. If you liked the others and the characters - like I do. Then you will like this one as well. It's pretty much the same laughs and the same morals for a third go round.
I would definitely recommend this to anyone who like the previous Wimpy Kid movies. I have read all the books and seen the movies and enjoy them for being simple, family movies that are not in 3D and don't go over the top. These are the kinds of movies that you can take your kids to and not worry if it's okay for them to see it.