The one that kicked them all off, Beach Party, was an enjoyable romp which seemed to even have a bit of a plot (gasp!). Well, they all have some kind of plot but this one is less convoluted and centers more on Frankie and DeeDee's relationship.
Basically, Frankie wants to get it on and DeeDee (Annette) doesn't. She wants him to see her more 'as a wife' so she invites the whole gang to their little beach hut when Frankie was expecting some 'alone time'. This upsets Frankie so they give each other the cool shoulder and spend their time flirting with others to make each other jealous.
One of the guys DeeDee flirts with happens to be an anthropology professor who happens to be studying the teenagers as they are clearly a different breed with their crazy lingo and swinging culture... and hijinx ensue.
Basically, Frankie wants to get it on and DeeDee (Annette) doesn't. She wants him to see her more 'as a wife' so... oh, well... it's pretty much the same concept as the first movie except this one has a load of 'muscle bound creeps' and an incredibly wealthy young Italian widow instead of a professor.
Obviously the attractive Italian widow who can 'buy' any man she wants turns her attentions to Frankie because he's such a catch. I can't really see the fuss.
These movies are full of the fashion, hairstyles and teen culture of the 60s. See them if you're looking for some light hearted fun and frivolity - avoid if you're after something intellectual.