Harry, Ron and Hermione start their third year at Hogwarts. The movie has the same people playing the characters, with no sudden cast changes (except for Professor Dumbledore). The new director does very well with all the special effects (Marauder's Map, Professor Lupin changing form, Dementors, etc.). But the story needs some additional explanation.

For instance, unless you read the book you might wonder: 1) Who are Mooney, Wormtail, Padfoot, and Prongs? And, 2) How does Sirius know that the Marauder's Map never lies? For those of you who would like the answers to those questions, here they are:

Question 1: The nicknames refer to what animals the wizards in question can transform into.

Mooney: Remus Lupin (a werewolf)
Wormtail: Peter Pettigrew (a rat)
Padfoot: Sirius Black (a dog)
Prongs: James Potter (a stag)

Question 2: Sirius helped to make the map when he was a student at Hogwarts.

The movie also shows Harry receiving the Firebolt broom at the very end, when in truth Harry gets it shortly after his Nimbus 2000 is destroyed by the Whomping Willow. It leaves out the fact that Sirius in the end signs Harry's permission slip to go to Hogsmeade, and gives Ron a new owl (since he lost him his rat), later named Pigwidgeon. The setting is still at Hogwarts, even though they changed a bit of the landscape.

To understand the movie the best, you should read the book before you see the movie. In my opinion the book is better, but hey, that's just me.

• Harry Potter official movie web site