Full disclosure: I have not seen "The Inbetweeners" TV show, for which this film serves as the series finale. I did, however, do extensive research (Wikipedia) prior to watching the film, and I'm fully aware there are some inside jokes I may have not picked up on.
Best friends Will (Simon Bird), Jay (James Buckley), Neil (Blake Harrison), and Simon (Joe Thomas) have a really hard life. And when we say "hard," we mean they live in an upper-class suburban neighborhood, their parents have really good marriages (except for Will's, but we'll get to that) and they've just graduated high school.
To catch you up to speed, Simon's girlfriend just dumped him, Will's dad just remarried his 22-year-old ex-wife (they only invited 100 close friends and family, and Will wasn't included) and Jay spends too much time masturbating to online porn using, yes, sliced ham as lubricant. This strange ritual also involves him wearing gloves, goggles and a snorkel. (Yeah, it gets weird.)
These three think their privileged lives currently suck and want to give adulthood a shot. So they (along with a bored Neil) plan a trip to Malia (paid in full by their parents) in hopes of getting (no surprise here) more vagina than Dirk Diggler (if you don't get that reference, just know it's a lot). These four teenagers are hungry for sex and manhood.
The goal of "The Inbetweeners" is to mock movie cliches while being a movie cliche itself. Stereotypes herein include (but are not limited to): bullies, road-trips-gone-wrong, mean principals, physical comedy, jerk dads, life lessons, drunken mayhem and an over-the-top ending. However, the main focus throughout is embarrassing, relentless and unsuccessful acts of sex. Some of the boys' standards drop drastically while Neil's completely vanish; take a moment and think of the most repulsive sex act you possible can and know that it's probably in the movie. Mission accomplished! (BTW, you have a very dirty mind.)
The only frustrating thing about "The Inbetweeners" is the boys' thick English accents — subtitles would have been incredibly helpful. An additional possible nitpick is their overbearing use of British slang, but we won't fault them for our lack of knowledge when it comes to foreign dialect.
That said, if you're a fan of the short-lived series, head to the theater (make sure to purposely sit in the the middle to be ironic) and see "The Inbetweeners." You're craving for cinematic filth will be fulfilled.