The basic plot of this movie is two brothers (the world's worst private detective and his wisecracking younger brother) are given a package to look after, but just about every major criminal in London is also after it.
This film is really a mixed bag. The story (based on The Falcon's Malteser, which is essentially a send-up of classic movies such as Casablanca and, of course, The Maltese Falcon) is great as it sticks pretty close to the book, unsurprisingly as the script was written by Anthony Horowitz, the author of the The Falcon's Malteser. Most of the actors/actresses are good especially the late Dursley McLinden, who plays Tim Diamond (real name Herbert Timothy Simple), Patricia Hodge (who plays Brenda von Falkenberg), Peter Eyre and Nickolas Grace (who play Gott and Himmel respectively), but unfortunately Colin Dale, who plays Nick Diamond (real name Nick Simple), and who is the main character, is hopelessly miscast.
Also, I'm not sure that Anthony Horowitz is that good at writing a script for a full length feature film (he has proved he is more than capable of scripting an episode of a TV show, as he has written for, amongst others, Midsomer Murders, Foyle's War and Agatha Christie's Poirot), but it may just be that The Diamond Brothers books (The Falcon's Malteser, Public Enemy Number Two, South by South East, The French Confection, The Blurred Man, I know what you did last Wednesday and The Greek who stole Christmas) just don't make for good adaptations, as a lot of the humour in the books is text based (one example from SbSE: "I can't say I'm a big fan of fun fairs. I can't afford the fares so I never have any fun").
I would recommend this movie, however, even to people who have never read the book, as it does not rely upon people having read the book to be able to understand it. It is entertaining, just don't expect it to be brilliant.