After a summer of isolation in the Muggle world, Harry Potter (Radcliffe) returns to the magic community only to discover he is the victim of mockery in the wizard media - few believe in the return of the evil Lord Voldemort (Fiennes), which he experienced first-hand at the end of year four.
On top of this, two lethal Dementors turn up in Little Whinging and attempt to give his loathed cousin Dudley (Melling) the 'kiss of death', and Harry is forced to banish them using the Patronus spell. However, being underage, Harry is not permitted to use magic outside of school and is promptly threatened with expulsion.