Pelias, the ancient king of Iolkos, overthrew his brother Aeson for the right to control all of Thessaly. Alcimede, Aeson's wife, feared for her infant son, Jason's, life so she hid him away with Chiron the centaur. Many years later after Jason had grown up, Jason returned to Iolkos to claim his place as rightful heir to the throne. Pelias told Jason that to claim the throne of Iolkos, he had to embark on a dangerous journey to obtain the famed Golden Fleece.
Jason set off aboard the Argo (hence Argonauts) with some of the bravest heroes in the land including Peleus, Heracles, and Castor and Pollux. The journey was indeed fraught with peril but along the way Jason received aid from unlikely sources. One of those sources, Medea, eventually became Jason's wife.
But the story of Jason and Medea does not end there. In fact their story is as magical as it is disturbing. Please do find Ovid's Metamorphosis and continue your reading.