This story spans 5 Constellations: Cassiopeia, Cepheus, Andromeda, Perseus, and Cetus.  Cepheus was the king of Æthopia, and Cassiopeia was his vain wife.  Cassiopeia felt that she and Andromeda, her daughter, were the

fairest women that ever lived, and voiced that they were fairer than even the Nereids, sea nymphs.  Hera, Zeus's wife, and the sea nymphs were furious, and they demanded Poseidon extract vengeance on Cassiopeia. Poseidon sent Cetus, the whale like sea monster, to ravage Cepheus' kingdom. Cepheus consulted the oracle, and it said he must sacrifice his daughter Andromeda in order to save his kingdom.  So Cepheus had his daughter chained to a rock by the ocean, and Cetus hungrily approached her.  Just as Cetus was about to finish Andromeda off, he felt a pain in his back, and he found that Perseus, flying on winged shoes, was stabbing him.  Perseus fought valiantly, but he was losing, until he remembered he had the head of Medusa in a bag.  He pulled out the snake haired head, and Cetus looked at it and turned to stone.  Andromeda fell in love with Perseus, of course, and they married.