The story of Perseus and Andromeda is told in Ovid's 'Metamorphoses' (4: 765-86). According to this source, Andromeda was the daughter of an Aethiopian king. She was chained to a cliff by the sea as a sacrifice to a monster from the deep. Perseus, who was also known for other heroic acts such as killing the Medusa, flew overhead, instantly fell in love with the princess, killed the beast and released her. Sometimes onlookers are shown watching from the shore.
Ariosto's story of Ruggiero's saving of Angelica, which occurs in his poem 'Orlando Furioso', has a similar plot and is often confused with this theme.