At the first meeting of the Slug Club, Harry sits nervously through a long, forced conversation with Professor Slughorn and various other students.

Harry, Ginny, and Neville leave Slughorn's compartment, discussing the Slug Club.
In the Slytherins' Compartment
Harry dons his Invisibility Cloak and follows [Blaise Zabini] into Draco Malfoy's compartment.
Harry overhears Malfoy, [Blaise Zabini], and [Pansy Parkinson] talking about the Slug Club.
Malfoy hints to his friends from Slytherin that he is working for Voldemort - with Harry listening in under his Invisibility Cloak.
Malfoy, left alone in his train compartment with an invisible Harry, performs the [Petrificus Totalus] on him, leaving him hidden and unable to move as the Hogwarts Express clears.
Chapter 8 - "Snape Victorious" - In which Tonks rescues Harry from the train and Snape leads Harry in to the feast, criticizing Harry all the way. Once inside, Harry discusses his status as 'The Chosen One' with those around him and Dumbledore gives his opening speech, announcing that Slughorn will be teaching Potions and Snape teaching Defense Against the Dark Arts. On the way out of the hall, Harry and Ron discuss Malfoy and run into Hagrid, who tells them about Grawp and leaves them feeling guilty for not taking his class.
Tonks to the Rescue
Harry lies alone on the floor of the Hogwarts Express, [Petrified] and invisible, thinking about how to get out of his predicament.
Tonks arrives, throws off Harry's Invisibility Cloak, and unfreezes him - they then jump from the Hogwarts Express and wait at the gates of Hogwarts for someone to arrive and let Harry in.
Late to the Welcoming Feast
Snape arrives to fetch Harry, criticizes Tonks, and goads Harry all the way up to the castle, taking off points for his lateness.