Harry Potter and the Prisoner of Azkaban
In my book, Harry, Ron and Hermione have ventured out to the Whomping Willow to stop Sirius Black. Lupin joins them, and explains that Black did not kill Harry's parents, but was set up by their other friend Peter Pettigrew, who is now Ron's rat Scabbers. Snape comes to get everyone in trouble, but the Harry, Ron and Hermione knock him out. Scabbers is forced to transform into Pettigrew, and everyone begins heading back to the castle. Lupin, who is a werewolf, sees the full moon and transforms. I think that Lupin will accidentally kill someone while he is a wolf, and Peter will get away from the group as a rat.