The third Harry Potter movie - The Prisoner of Azkaban - starts out strong. An angry Harry unknowingly blows up Aunt Marge by using an Inflating charm on her and storms out of the Dursley household not sure where to go next. The Knight Bus picks him up, and its conductor, Stan Shunpike informs him that a mass murderer named Sirius Black is on the loose. Harry reaches Leaky Cauldron where two more people, the Minister of Magic, Cornelius Fudge, and an employee at the Ministry, Arthur Weasley advise him to act responsibly. The third Harry Potter movie builds tension through exciting and suspenseful scenes and their payoff comes at the Shrieking Shack, where old friends reunite and bring shocking news to the Trio.
Harry Potter and the Prisoner of Azkaban also introduces an interesting character named Professor R.J. Lupin on the Hogwarts Express. The Trio shares a nearly-empty compartment with him because everywhere else is full. He's shabbily dressed, his face is hidden under tatty wizard robes, and he's fast asleep. Harry takes advantage of the situation and tells his friends everything he has been told about Black's prison break. Lupin finally wakes up when a Dementor checks for Black at the threshold of their compartment. He looks visibly ill, his hair is unkempt, and upon reaching Hogwarts, Harry, Ron, and Hermione learn he is to take up the Defense Against the Dark Arts with them.
Remus Lupin's Ordeal
Lupin and Harry bond gradually, specifically after the day he honors the late Lily Potter's memory in his presence. He describes his mother as a kind woman who was there for him when no one else was, adding, "She had a way of seeing the beauty in others, even, and perhaps most especially, when that person couldn't see it in themselves." Lupin's words and body language make it clear that he hides behind his smile and pleasant behavior a deep dark secret, and suffers in pain. In Harry Potter and the Prisoner of Azkaban's pivotal scene - the encounter at the Shrieking Shack - Hermione tells her friends that their Defense Against the Dark Arts Professor has been missing classes because he is a Werewolf. Lupin asks her how long she has known, and she replies by saying ever since she wrote Professor Severus Snape's essay on Werewolf and how to recognize it.
Turns out, Hermione is correct, as shortly after, she and her friends, and Black (wrongly believed to have betrayed the Potters to Lord Voldemort), witness Lupin turn into a werewolf as they make their way out of the tunnel that leads to the Shack. Chapter 20 of Harry Potter and the Prisoner of Azkaban, "The Dementor's Kiss" lays out in detail Lupin's Werewolf transformation. It happens when he and the party tramp through the grounds. Black freezes as he sees his friend's limbs shake and signals Harry and Hermione to stop. Since a full moon graces the skies, Lupin, who hasn't taken his Wolfsbane potion, begins to transform. The man has no choice in his transformation, and once he turns into a werewolf, Black transforms into a dog to guard the kids against this Werewolf's wrath.
Fenrir Greyback and Lyall Lupin
Remus Lupin's ordeal began at the age of four. He was turned by a werewolf named Fenrir Greyback who sought revenge on his father, Lyall for making disparaging remarks about the Werewolf community. Greyback bit and contaminated the only son of Lyall, Lupin, thereby afflicting him with lycanthropy for life. Greyback wanted to turn as many children as he could into werewolves with the purpose to build an army of Werewolves that could take over the Wizarding community. Lycanthropy in Harry Potter lore is a magical illness, known to occur when saliva and blood come into contact. In other words, when a transformed werewolf bites a human, the bitten becomes a werewolf, as seen in the case of Remus Lupin. In Harry Potter and the Half-Blood Prince, Professor Horace Slughorn mentions to Harry, Neville Longbottom, Hermione, and the rest that Marcus Belby's uncle, the potioneer, Damocles invented the Wolfsbane Potion. Though this complex potion is known to relieve the symptoms of lycanthropy, it cannot cure the affliction.
After Lupin's secret is out, he decides to resign from his position and leave Hogwarts. In the last chapter of the third Harry Potter book - The Prisoner of Azkaban "Owl Post Again," Harry sees him in his office. It turns out, Severus Snape outed him to Slytherins at breakfast and quite obviously, this news spread like wildfire. "Professor Dumbledore managed to convince Fudge that I was trying to save your lives," says Lupin, adding that Dumbledore's move was the last straw for Snape. In the end, Lupin leaves Hogwarts for the safety of the students, and he deems it to be reasonable. Dumbledore informs him his carriage has arrived at the gates, and with that, the thoughtful man bids his D.A.D.A. teaching position goodbye.