Harry Potter: The story so far. Book 3
Harry realises he is very vulnerable against dementors -- every time one is near him, he starts reliving the night Voldemort killed his parents.
Professor Lupin comes to his rescue, and teaches him how to do the Patronus Chram, by which a silvery wisp in the form of an animal emerges from a wizard's wand and drives away dementors.
But Professor Lupin also has a secret.
Meanwhile, Harry keeps seeing a huge dog watching him.
Also Read: The magic of Harry Potter