The oracle, or prophecy, that was delivered to King Acrisius said that he will be killed by the son of Danae. Since Danae is his daughter, King Acrisius will be killed by his grandson. This is especially ironic, because the king sought the oracle in the first place due to his disappointment in not having a son.
What I do when I have a report and don't read the book, I go to google and look up a summery for the book, it should only be a few paragraphs that tells the whole story.
