some of the backstory of the marauders.

 James, Sirius, peter and remus were all best friends at Hogwarts, they were also animagi (James could turn into a stag, Sirius was a black dog, peter was a rat and remus was a werewolf. they became animagi to keep remus company on full moons.)Sirius was the secret keeper for lily and James's location during the war to keep them safe from voldemort, but he changed it to peter because he figured that if Voldemort wanted to know where they were, he would think that sirius knew. After peter became the secret keep he told Voldemort. James Potter, lily Potter and harry Potter were a happy family before peter became a coward and joined Voldemort. He became the spy for Voldemort in the oder of the phoenix and sold lily and James to Voldemort to kill them. When Voldemort arrived to kill them lily cast a spell on harry to protect him giving her life for him. Once Sirius realized that peter told Voldemort he chased after peter to try to kill him but to frame Sirius, peter cut off his finger, made an explosion, turned into a rat and ran away to make it seem like Sirius went on a rampage. Sirius got locked up in Azkaban fro 12 years because of it until he finally escaped 12 years later and that is when "Harry Potter and the prisoner of Azkaban" started and harry was about 13. That is why Sirius seemed like the bad guy in that movie because nobody gave him a trial before sending him to jail. Everybody believed that he really was the one to sell lily and James to Voldemort when it was peter.