Akrisios was told by an oracle that he would be killed by his grandchild. Because of that he put his daughter Danaë in a tower where noone was allowed to enter, so she could not have a child. But the god Zeus could enter in the form of a golden rain, and with him her child was Perseus.
Akrisios then put Danaë and her baby Perseus in a box and threw it into the sea. But Zeus tells the sea-god Poseidon to help them, and so Danaë and her son come to the island Seriphos. There they are found by Polydectes, who lets them live with him.
A long time after that, the grown up Perseus takes part in some funeral games. During the games Perseus throws a discus, which accidentally hits Akrisios. Akrisios is killed by it: he was killed by his grandchild, like the oracle's prophecy said.
Akrisios Facts for Kids. Kiddle Encyclopedia.