Ron receives a Howler from his mother, berating him for taking the car.
His strategy at Wizard's Chess allows Hermione and Harry to proceed safely through a dangerous life-size, animated chess game.
During the game, Ron allows his piece to be sacrificed and is subsequently knocked unconscious.
Harry and Hermione follow the tunnel, which leads to the Shrieking Shack.
The dog is actually the animal form of Sirius Black (an Animagus), Harry's godfather and an escaped convict from the wizard prison Azkaban.
Ron offers to help with the preparation of Buckbeak's defence, but this fails to help.