Confusing! That's what I thought after reading the third Harry Potter book. That's the main trouble with time travel. If you want to have a time travel story, you need to make sure everything make sense while the action is happening and in the end.
That's the main problem with the time travel parts of this book. It makes sense in the end...only if you are willing to make a leap of faith in the midst of the story.
To be fair, the story was pretty good till the time travel started. J.K. Rowling's third book in the Harry Potter series opens with Harry Potter enduring yet another unhappy summer at the Dursleys'. Then Uncle Vernon's visiting sister, Marge, viciously insults Harry, Harry's anger snap and magically causes her to inflate and float to the ceiling. Harry runs away but the Knight Bus soon find him and takes him to the Leaky Cauldron.
During the trip, Harry learns that a supporter of Lord Voldemort, Sirius Black, has escaped from the magic prison of Azkaban. Black murdered thirteen people with one curse and is a strong supporter of Lord Voldemort. Harry met with the Minister for Magic, Cornelius Fudge, who surprising told Harry that he will NOT be expelled from Hogwarts for using under-age magic. While staying at the Leaky Cauldron, Harry hears Mr. and Mrs. Weasley arguing over whether he should be warned about Black.
As Harry begins his third year, there are two new wizards joining the staff: Professor Remus J. Lupin for Defence Against the Dark Arts and Rubeus Hagrid for Care of Magical Creatures. While Lupin's lessons are enjoyable, Hagrid's soon become dreary. The hippogriff Buckbeak, a half-horse, half-bird creature, was provoked into attacking Draco Malfoy whose father, Lucius Malfoy, immediately files a complaint against Hagrid.
Because Black is still at large, Dementors, the inhuman Azkaban prison guards now patrol the school. Dementors drain happiness from anything they approach and Harry find that he is particularly affected by them.
Shortly before Christmas, the Weasley twins give Harry their Marauder's Map, a magical document that shows every person's location within Hogwarts as well as secret passageways in and out of the castle. Harry uses a tunnel to sneak into Hogsmeade village where he overhears a disturbing conversation that Black was his parents' best friend and is his godfather and legal guardian. He was also the one who betrayed the Potters whereabouts to Lord Voldemort and murdered their friend Peter Pettigrew.
A large dog then attacks Ron and drags him and his rat Scabbers into a hole at the base of Whomping Willow. Harry and Hermione follow, finding a tunnel leading to the Shrieking Shack. Inside, Harry confronts Sirius Black, who is an Animagus, a mage who can transform into an animal at will. Lupin suddenly bursts in and embraces his old friend Black. Confronted by Hermione, Lupin admits to being a werewolf and also the Map's creator, along with Black, Pettigrew, and James Potter. Lupin and Black explain that Scabbers is actually Peter Pettigrew in his Animagus form. He is Voldemort's servant, and he betrayed the Potters, framing Black for the crimes. Harry is skeptical until Black and Lupin force Pettigrew back into his human form. Black explains he discovered that Pettigrew was still alive and escaped Azkaban to seek revenge.
As the group heads back, the full moon rises, causing Lupin to turn into a werewolf. During the ensuing commotion, Pettigrew escapes. Black turns into his dog form to protect the others from Werewolf Lupin. Lupin flees, leaving Black badly injured. As Dementors move in to attack Black, Harry and Hermione see a mysterious figure in the distance cast a powerful stag-shaped Patronus, a guardian spirit spell. The stag scattered the Dementors. Black is then captured and taken to the castle where the Dementors intend to suck out his soul.
Hermione then reveals to Harry that she was entrusted with a time-traveling device. Prompted by Headmaster Dumbledore, she and Harry travel three hours into the past, watching themselves go through the night's previous events.
This is where the story start losing sense. Harry and Hermione set Buckbeak free and return to the Whomping Willow. As the dementors are about to attack the "other" Harry and Black, Harry realises that the mysterious figure he saw earlier was actually himself. He casts the powerful Patronus that repels the dementors. Harry and Hermione free Black, who escapes on Buckbeak as the timeline restores itself to normal.
Does the last part make sense to you? It sure didn't to me as I was reading it. Hermione just happen to be entrusted with a time travel device, which is a big thing in the magic world. Harry just happen to learn the Patronus spell which save Black. They then managed to free Buckbeak who flies Black to safety.
Coincidence? No such thing, magic time travel! A good book that tries so hard to find a good ending, it suffers. That's my view.