Allison's Aunt Kate comes into town and is attacked by the Alpha Wolf. When she attempts to kill it, she finds Derek Hale instead, and shoots him. She used a magic bullet that would have killed him in 48 hours, but Scott found that same type of bullet and helped Derek save his own life.
Scott found the bullet at the Argent home, when he went over for dinner. He originally was there alone with Allison, but Chris and Kate came home, and the wacky Aunt invited Scott over. After it got all kinds of awkward between Scott and Chris, the young werewolf snuck off to look for the bullet. Almost getting caught, Scott finally found the bullet and made it back to Derek.