Shakespeare didn't come up with this story all on his lonesome. (Don't call the plagiarism brigade just yet: most of Shakespeare's plots are borrowed, and people in the 16th century didn't think about "originality" in the same way we do.) The story of Hamlet dates back to at least the 9th century. It centers on "Amleth" (sound familiar?), a young man who fakes being crazy in order avenge his father's murder. Saxo the Grammarian included the tale in a 12th century text and later, François de Belleforest translated the story from Latin into French in Histoires Tragiques (1570), which is where Shakespeare may have found it.