As they enter the castle, Harry and Hermione are called away by Professor McGonagall. She and Madam Pomfrey make sure Harry is alright after the [dementor] attack, and then after McGonagall has a brief private conversation with Hermione, they all make their way back to the Great Hall together.
On the Hogwarts Express
A frenzied breakfast ends with the Weasleys, Harry, and Hermione taking [Ministry cars] to [Kings Cross Station], while Harry continues to think about Sirius Black and Ron protects Scabbers from Crookshanks.
Arthur Weasley and Harry stroll onto [platform nine and three-quarters], and are soon followed by the rest of the family. Before saying goodbye, though, Harry admits to Arthur that he'd overheard his conversation about Sirius Black, and runs off to catch the train as it pulls out of the station.
Harry pulls Ron and Hermione into a private compartment, with only a sleeping Professor Lupin already there, and tells them about how Sirius Black is after him. After a brief interruption from Harry's Sneakoscope, they discuss instead the fact that Harry won't be able to visit [Hogsmeade].
As the Hogwarts Express continues on, the [witch with the food cart] comes by and is followed shortly by Malfoy, Crabbe, and Goyle, though they decide it foolish to attack Harry with Professor Lupin in the compartment.
The Affects of the Dementor
The Hogwarts Express suddenly stops and goes dark, and after some apprehensive moments in which Ginny and Neville arrive, a [dementor] comes into Harry's compartment - causing Harry to hear a scream and black out.
The feast begins with announcements from [Dumbledore]: the [dementors] will be guarding Hogwarts until further notice, Remus Lupin will be the new Defence Against the Dark Arts professor, and Rubeus Hagrid will be taking over Care of Magical Creatures.