Don't Panic
Indeed, there is no reason to panic, for this movie is a faithful adaptation of the books. There are several scenes thrown into the movie that I thought weren't really needed, and were really just an excuse for some slapstick comedy and a romance plot that was virtually absent in the book. Still, through passages from the book being read aloud, the spirit remains wholly intact.
So you think you're having a bad day? Meet, Arthur Dent (Martin Freeman), an englishman who happens to reside in a house that is scheduled for demolition so a highway can run over it. This is of little concern to his best friend Ford Prefect (Mos Def), who isn't a human at all, but a humanoid from a planet in the vicinity of Betelgeuse. It turns out he's an interstellar hitchhiker, who's been stranded on earth for the past 15 years. Well, he knows that earth is about to be destroyed to make way for an interstellar highway. They hitch a ride with the destructor fleet, but find themselves soon after on the Heart of Gold, with President of the Galaxy, Zaphod Beeblebrox. He just so happens to be looking for Magrathea, which supposedly is home to a computer with the answer to life... the universe and everything.
You must realize that the story is secondary to the wit. The Hitchhiker's Guide (H2G2), was written as a parody to all the serious sci-fi stories out there. There aren't any epic battles or dog fights in the void of space. There are, however, clinically depressed robots and horrible poetry. I felt that much of the humor in the book was literary, so thankfully, a narrator has been included in this film. That means that great passages from the books such as the dolphins and the sperm whale remain fully intact. It seems that a majority of the humor in the film is slapstick, and while I would've prefered the dry wit of the book, it works just as well. The ending has also been tampered with, and although it is very funny, it is very different.
I would suggest that you read the book by Douglas Adams first before you see this film. You're appreciation will be a little bit deeper, and so will your knowledge of what exactly is going on. Still, if you don't have that option, the movie is worth a view. It's pretty funny, and a lot of fun.