Harry Potter and the Prisoner of Azkaban Joanne K Rowling
Sirius
Black has escaped from Azkaban! The wizard who helped murder Harry's parents
is on the run. Harry is homeless after argueing with the Dursleys. On the
train to Hogwarts, he meets Professor Lupin and the dreadful dementors,
who are this year protecting Hogwarts against Sirius' possible attempts
to kill Harry. It is only later on that we discover that all is not what
it seems at first.
Hunted
by the Grim, Harry finds shelter in Hogwarts after a difficult first encounter
with the Dementors. He finds comfort in the company of Lupin, who reveals
to him that he knew his parents well. The discovery of the Marauder's Map
suddenly brings forward a secret and a man back from the dead, whereas his
encounter with Sirius Black is not what he expected at first.
More would be a spoiler - read the book (or watch the film)
Harry Potter films
The Harry Potter films were a success from the start - according to me
because the - then - children cast for the roles match very well. Even
now, with his teenager voice, David Radcliffe (Harry Potter) matches the
slow evolution of Harry into a teenager conscious of the stakes he is
involved in.
Overall, although for instance in The Prisoner of Azkaban the film has
difficulties to follow the slow, tedious building of the plot, the story
remains understandable - obviously much more if you have read the books.