After the war, Harry's personality was irritable and abnormal, and the changes in many of his friends made him even more uneasy. It was not until after Christmas Eve that he accepted Hermione's request: to hunt down Draco, the fugitive from Azkaban.
It's like a Quidditch game. What Harry has to do is no longer find the Golden Snitch and win honor for Gryffindor, but capture the platinum head.
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