Well, as you know, he's the first person ever to escape from Azkaban. But you don't know the real reason he did it. When you defeated You-Know-Who, since he was his most faithful servant, Black lost everything. And now...now he's out to get you...and kill you!
Harry, I must tell you something. Promise to keep it a secret, there are many who wouldn't want me to tell you this - it's about Sirius Black!
Why are we stopping?! We can't be there yet?
Is it me or has it suddenly become cold in here?
Why is he not waking up? What is that?!
...but Black is Harry Potter's godfather. He was the one who told You-Know-Who where Lily and James Potter were hiding...
We couldn't believe he betrayed them like that. He killed Peter Pettigrew as well...
That poor boy!
After leaving his aunt and uncle's house and spending a few weeks in a wizarding pub - the Leaky Cauldron, Harry, Hermione and the Weasleys are at the train station to board the Hogwarts Express and begin their journey to Hogwarts. However, right before the train departs, Mr Weasley pulls Harry aside for a quick conversation - one that turns Harry's inital excitement about the new school year into fear.
Scabbers, no!Hermione, he bit me!
Once inside the train, Harry, Ron and Hermione find a sleeping man inside their compartment. The name on his suitcase reads: Professor R.J. Lupin, meaning that he is their new Defense Against the Dark Arts teacher. Once they settle in, Harry tells his friends about Mr Weasley's warning. Before they can discuss it, the train stops. Darkness and panic invade the space, strange creatures roam the train and Harry faints. Professor Lupin finally wakes up and produces a spell that chases the dark creatures (dementors) away.
The truth about Sirius Black
Sirius, what's happening to professor Lupin?
Take Ron to the castle. I'll help Remus!
After realising he can't leave the castle without meeting a dementor, Harry puts on his invisibility cloak and sneaks into Hogsmeade - a wizarding village third year students are allowed to visit if they have their guardian's permission (which Harry doesn't). When he accidentally enters Madam Rosmerta's pub, he overhears a conversation between professor McGonagall, Minister Fudge and Rosmerta that makes his hatred for Black much greater than it was before.
Help! I'm innocent, help me!
Harry, Ron and Hermione go to support Hagrid through a hard time, but they face the same bad omen Harry's been seeing all year - the Grim. Meanwhile, Ron's rat Scabbers bites him and flees. As he chases after Scabbers, Ron gets bitten by the Grim. Harry and Hermione go to help him and come face to face with Sirius Black! After a year of being chased by both muggles and wizards, he reveals his secret to Harry - he's an Animagus whose animal form is a dog. All that's left to do now is for Sirius to prove his innocence to Harry.
Harry, run! It's the Grim!
As Sirius tries to explain himself, professor Lupin comes to the scene and realises what happened at once. Together, Lupin and Sirius tell the trio of the what happened 13 years ago. Because they are distracted, Pettigrew transforms (he's Scabbers, another Animagus) and escapes wich goes unnoticed until it's too late. Another issue arises: Lupin sees the full moon and the impact is immediate - his transformation into a werewolf happens rapidly, and Sirius distracts him to give the kids a chance to get to the castle.
Scabbers is running away!
Once they leave Ron in the school hospital, Harry and Hermione use her Time-Turner to go back in time and change their actions in order to save Sirius from the Dementor's Kiss. They are successful in doing so and Sirius gets away. A few days later the Firebolt (the best quidditch broom on the market) arrives to Harry by owl post with a letter from Sirius. Even though a bit discouraged by Pettigrew getting away and Lupin being forced to resign, Harry is a bit more peaceful leaving for the summer holidays.
Hermione, what is this?!
This is a Time-Turner, Harry. Professor McGonnagal gave it to me on our first day back. All year I've been using it to manage my classes and now we're going to use it to rescue Sirius!