Plot to murder Peter Pettigrew

During the 1993–1994 school year, Sirius Black engaged in an attempt to kill Peter Pettigrew and avenge the deaths of James and Lily Potter. Despite being hindered by being a fugitive from the Ministry of Magic and Peter being well-protected inside Hogwarts Castle disguised as a pet rat owned by Ronald Weasley, Sirius eventually managed to get Peter away from the castle and to the Shrieking Shack. However, in the end, Peter escaped, and would contribute to his master's return almost a year later.

Peter's betrayal of the Potters
"I persuaded Lily and James to change to Peter at the last moment, persuaded them to use him as Secret-Keeper instead of me...."

- Sirius Black discusses the Secret-Keeper gambit.

When James and Lily Potter prepared to go into hiding from Lord Voldemort in October of 1981, they decided to put a Fidelius Charm over their home as part of their protection. Friends such as Albus Dumbledore and Sirius Black offered to be their Secret-Keeper, but they ultimately decided that a more unlikely choice would be safer, and picked Peter Pettigrew. Unbeknownst to them, Peter was a double agent actually working for Voldemort, and quickly turned the location of the Potters over to his master. Though he would be unsuccessful at killing Harry Potter due to Lily's sacrificial protection and was reduced to a mere broken soul without a body for over a decade, Voldemort did managed to kill both James and Lily thanks to Peter's betrayal.

Sirius's first attempt at revenge
"I — I will never forget it. I still dream about it sometimes. A crater in the middle of the street, so deep it cracked the sewer below. Bodies everywhere. Muggles screaming. And Black standing there laughing, with what was left of Pettigrew in front of him...a heap of bloodstained robes and a few — a few fragments."

- Cornelius Fudge's incorrect assumptions of what happened that morning.

The following day, 1 November, Sirius cornered Peter on a street in London, intending to kill Peter for his betrayal. However, Peter was too quick and set off a Blasting Curse, killing twelve innocent Muggles and creating a diversion, allowing Pettigrew to cut off one of his fingers, take on his rat form, and escape. As this left a large amount of incriminating evidence against Sirius, he was arrested and sent to Azkaban for the next twelve years. Peter would later make his way into the ownership of the Weasley family as a pet rat, and was owned by Ronald Weasley by the time of Sirius's escape.

Sirius's escape and trip to Hogwarts
"He's at Hogwarts...He's at Hogwarts..."

- Sirius Black obsessing over Peter Pettigrew.

Unlike most inmates at Azkaban, Sirius managed to keep his presence of mind by focusing on his desire to kill Peter, a thought that wasn't happy enough to be taken from him by the dementors, but gave him something to focus on other than depression. Sometime in late July, Sirius bummed a paper off a visiting Cornelius Fudge and saw a story on the Weasleys winning the Daily Prophet Grand Prize Galleon Draw. Immediately recognizing Peter's rat form in the image, Sirius was emboldened and slipped past the dementors in his dog form, swimming to shore, only taking a short detour to Little Whinging to catch a glimpse at Harry before heading to Hogwarts and wait.

Attempt at Hallowe'en
After lurking around the castle grounds for a while, Sirius made his first attempt on 31 October, correctly assuming that most of the staff and students would be in the Great Hall having the traditional Hallowe'en feast. Though he made it as far as the Fat Lady, she refused to admit him without a password. In a rage, Sirius made a vain attempt to slash his way into the common room with his knife, frightening off the Fat Lady, before quickly making his escape. He managed to elude the staff's total search of the castle and grounds for him, and went back to planning his next attempt.

Attempt in Feburary
Sometime afterward, Sirius met Crookshanks, the pet cat of Hermione Granger. After communicating that he wanted to kill a rat belong to Ronald Weasley, Crookshanks set off for Gryffindor Tower. Though he failed to capture Peter, Crookshanks did bring back a list of passwords that Neville Longbottom had gotten from Sir Cadogan. Armed with this, Sirius once more broke into the castle, taking off to the Gryffindor Boys' Dormitory and standing over Ron's bed. Unfortunately, he found that Peter had gotten wise and faked being eaten by Crookshanks, and thus was no longer in the castle. Ron woke up and yelled, and Sirius only barely escaped before the staff showed up.

Final attempt


Eventually, on 6 June, Sirius saw that Ron, along with Harry and Hermione, had managed to find Peter again. Though Peter attempted to flee, Sirius attacked Ron, carrying them both to the passage beneath the Whomping Willow to the Shrieking Shack, with Harry and Hermione following quickly behind with Crookshanks's assistance. Remus Lupin, who saw this all happen on the Marauder's Map he had confiscated from Harry earlier that year, would follow after a short time.

After a struggle, Sirius and Remus, the latter only fully understanding after seeing Peter on the Map, explained to Harry, Ron, and Hermione the above events, and managed to convince them that Sirius was innocent of the crime he was convicted for. Before they could reveal Peter, however, Severus Snape, who had seen Remus heading for the Whomping Willow on the Map when coming to give him his Wolfsbane Potion, arrived, hidden under the Cloak of Invisibility Harry had left outside the Willow. He refused to believe Sirius's story and was only stopped from taking him to the dementors by a triple Disarming Charm from Harry, Ron, and Hermione, knocking him unconscious.

After this diversion, Remus and Sirius revealed Peter and proved Sirius's story. Though they were fully prepared to kill their former friend, Harry interjected at the last moment, saying that James wouldn't have wanted his friends to become murderers, and instead opting to offer Peter up to the dementors.

Aftermath
Unfortunately, this this not go as expected. That night happened to be a full moon, and Remus, who was chained to both Peter and Ron, took on his werewolf form. In the resulting chaos, Peter escaped once more and Sirius's name was not cleared. Indeed, only the clever actions of Hermione and her Time-Turner saved Sirius from the Dementor's Kiss. As prophesied by Sybill Trelawney earlier that night, Peter would go on to rejoin Voldemort, and was a major contributor in his rebirth almost one year later.

Appearances

 * Harry Potter and the Prisoner of Azkaban
 * Harry Potter and the Prisoner of Azkaban (film)
 * Harry Potter and the Prisoner of Azkaban (video game)
 * LEGO Harry Potter: Years 1-4