Acrisius was disappointed that he had no sons to give his throne, and asked an oracle for help. The answer he got was that his grandson would kill him. At that point, Danae was childless, and to keep the prophecy from coming true, Acrisius locked her in a tower. However, Zeus had seen Danae and had grown fond of her; so, during the night, he appeared to her in the form of golden rain and impregnated her. She gave birth to a baby, which she named Perseus.
When Acrisius found out what had happened, he was infuriated; however, he did not want to kill his grandson out of fear of the Furies. So, he locked Danae and Perseus in a chest and threw it at sea. The chest drifted away and reached the island of Seriphos, where Dictys, a fisherman and the local ruler's brother, helped the mother and the child. The king of Seriphos, Polydectes, fell in love with Danae and tried to forcefully marry her. However, Perseus did not let him. Furious, but also not wanting to outright kill Perseus, Polydectes said that he would stop pursuing his mother if Perseus would slay Medusa and bring back her head. So it happened, and Danae was saved.
When Perseus reached adulthood, he became a great hero and managed to accomplish a number of feats. He eventually decided to return to Argos and see his grandfather, but after finding out about the prophecy, she changed course and went to Larissa in order to participate in the athletic games that were held there. What he did not know, though, was that Acrisius also attended the event. While Perseus was throwing the discus, an accidental misthrow caused the discus to land on his grandfather's head, and resulting in his instant death. Thus, the prophecy was fulfilled.