The princess was tied to a tree to be sacrificed to the monster. Luckily Perseus was out adventuring and saw the beautiful woman. He fell in love immediately. He quickly ran ashore and freed her of the chains. The monster then snapped and just as fast as he came he chopped off the monsters head.
Perseus and Andromeda went back to the King. Perseus said that if he let him marry his daughter that he would kill the monster. The king thought about this and agreed. He liked that Perseus was half god and his father was Zeus he couldn't be more happy.
In the end they got married and the kingdom lived happily ever after.