**spoilers for Harry Potter and the Prisoner of Azkaban to follow-- do not read past first paragraph if you haven't read the book and don't want to know what happens-- if you only don't want the big secrets revealed to you, then you can probably read to the end of the second paragraph-- but that's it! big important surprises after that if you haven't read :) enjoy**

In book 3 of J.K. Rowling's Harry Potter series, Harry Potter and the Prisoner of Azkaban, a new character is introduced: the convicted criminal Sirius Black. At first, all Harry knew about him was what he overheard Ron's parents say about him, i.e. that he was dangerous and had been heard in his prison cell in Azkaban to be muttering, "He's at Hogwarts..." over and over. Even though Harry doesn't know why Sirius was in prison in the first place, he realizes of course that the Weasley parents are especially worried for him because Sirius has escaped from Azkaban, something no other prisoner has ever been able to do before, and may be coming after Harry.

As Harry finds out later, from another overheard conversation since no one wants to forthrightly tell him about all this, Sirius was convicted on charges of murder and accomplice to murder: 13 counts of Muggle murder (with one curse), 1 count of wizard murder (although all they could ever find was a finger, not the body), and accomplice to 2 murders-- those of Harry's parents. Apparently, Sirius had been James and Lily Potter's best friend, even best man at their wedding, and he became Harry's godfather when he was born. So when they were on the run from Lord Voldemort, the Potters naturally made Sirius their Secret Keeper so that only he would know their location. But their Secret Keeper betrayed them, and Lord Voldemort found the Potters and killed them, while Harry escaped the deadly encounter with a small souvenir, a thin lightning-bolt-shaped scar on his forehead that throbs and hurts and tortures him sometimes.

After Harry hears all this, he doesn't know that there's still quite a bit of story left to tell, but he finally finds out at the very end of the book in an encounter between Harry, Hermione, Ron, Professor Remus Lupin, Professor Snape, Sirius, and believe it or not Peter Pettigrew.

As it turns out, Sirius had convinced the Potters to use another friend as their Secret Keeper, unwittingly leading them straight into the hands of a follower of Voldemort, Peter Pettigrew. Pettigrew had been not really a friend of Sirius and the Potters, but more of a hanger-on, a weak follower-type, which is why he was so easily converted to the dark side. When he saw the chance to deliver the Potters to his master, he took the opportunity, little knowing that the incident with the infant Harry would lead to the near-extinction of Voldemort.

Sirius was the only one who knew that it was Pettigrew who had betrayed the Potters, and he confronted him about this in public as the authorities were trying to capture Sirius. However, he hadn't counted on such desperate cunning as Pettigrew showed when he, in one deft movement, killed 13 bystanders with one curse and at the same time cut off his finger and turned himself into a rat. The point of this was to make it appear as though Sirius was indeed the dangerous criminal who had betrayed the Potters, and he was crazy enough to take out 14 more people when cornered. What's all this about Pettigrew the rat, and 14, not 13, people? If you're confused, be patient and read on.

Sirius spent years in the dread prison of Azkaban for his alleged crimes, and when he finally got out, he wanted to have his revenge upon Pettigrew, who he knew was at Hogwarts ("He's at Hogwarts..."). How did Pettigrew manage to get there, you ask? Follow this semi-side note and you will learn:

Professor Lupin, the students' Defense against the Dark Arts teacher that year, had been a friend of the Potters and Sirius when they were all in school together. Unbeknownst to the rest of the wizarding world, Lupin, James Potter, Sirius, and Pettigrew were all unregistered Animagi, i.e. they could turn themselves into animals. Lupin was a werewolf (nickname Moony), James Potter could turn into a stag (Prongs), Sirius into a large black dog (Padfoot), and Pettigrew into a rat (Wormtail). **fun fact: Remember these names? Padfoot, Prongs, Moony, and Wormtail are the creators of the Marauder's Map!** Now remember our good friend Ron Weasley? He has a pet, Scabbers, a scrawny little... rat. Yes, rat. That's right, Peter Pettigrew aka Wormtail had been living in disguise as Ron's pet rat ever since he disappeared in that confrontation, leaving part of his body (a finger) to make it look like Sirius had killed him with that explosive spell.

This all came out in the aforementioned encounter that happened at the very end of the book. Now what happened after everyone there learned the truth about Sirius Black? Long story short, Wormtail escaped, Sirius couldn't prove his innocence without him, and Harry's joy at finding his godfather to be a good person and a great replacement for the Dursleys was cut short as Sirius remains on the run from the Ministry of Magic. Confused? Intrigued? Want to know more (and there is so much more)? Read the book. It's fabulous.