Chapter 17 - "Cat, Rat, and Dog" - In which HRH start back to the castle, Crookshanks chases Scabbers, and a big black dog drags Ron into a tunnel under the Whomping Willow. Crookshanks stops the tree from attacking Harry and Hermione, and they follow the cat down the tunnel to emerge in the Shrieking Shack outside Hogsmeade. They find Ron a captive of Black, who disarms them. Harry attacks Black, and with Ron and Hermione overpowers him, but Crookshanks stands between Harry and Black when Harry threatens to kill him. Lupin arrives and admits that he's Black's friend and a werewolf. It is revealed that Scabbers is an Animagus named Peter Pettigrew.
Professor Trelawney Strikes Again
Harry and Ron make their way up to their final exam, Divination, at the top of the North Tower. They wait for a time at the bottom of the ladder, muttering about the suspiciousness of the exam, before Harry finally gets called in.
Harry sits his Divination exam, which passes rather unsuccesfully for him. However, at the end Professor Trelawney turns very strange, and in a harsh voice predicts that the Dark Lord's servant will set out to rejoin him that same night. When she comes out of her state, she has no memory of the prediction, and Harry takes off.
Harry runs back to Gryffindor Tower to tell Ron and Hermione about Trelawney's strange prediction, but stops short when they tell him that Buckbeak is set to be executed. They decide to go down to visit Hagrid, and to help, Hermione runs off to grab Harry's invisibility cloak, impressing Ron.
In Hagrid's hut, Harry, Ron, and Hermione talk to him about Buckbeak's execution. Unexpectedly, Hermione finds Scabbers there, but Hagrid soon sees the executioner coming and shooes them, hesitating, out the back door.
It's a Trap!
Not yet back in the school and upset as they listen to Hagrid's howls, Harry, Ron and Hermione's situation quickly turns to chaos. Scabbers bites Ron and escapes, with Ron leaving the invisibility cloak to catch him; the giant dog grabs Ron by the arm and drags him under the Whomping Willow, and Crookshanks stops the tree from slashing Ron and Hermione, as all three chase after the dog and Ron.
Sirius Black begins talking to Harry, Ron, and Hermione, and overcome with outrage, Harry attacks him physically. He wins the fight and ends standing over Black with his wand pointed at him, but before Harry can bring himself to do anything, Lupin arrives and disarms him, leaving Harry thinking that Black would be given over to the [dementors].