Pelias has usurped the crown that rightfully belonged to Jason's father, and when Jason comes of age, he returns to claim the kingdom from his cousin, Pelias. Years before, Pelias had received a prophecy that he would be killed by a member of his own family and that he ought to beware of a man that comes to him wearing only one sandal. Jason, his cousin, does arrive wearing only one sandal, though he is resplendent in every other way.
Jason tells Pelias that Pelias may keep all of his cattle and land but he must turn over the throne, by right, to Jason. Pelias agrees, but he says that, first, Jason must go to Colchis to retrieve the Golden Fleece for him. Jason is young and strong while Pelias is old and frail, and so Pelias claims that he cannot go himself. Secretly, however, Pelias believes that Jason will never survive the perilous journey and so he will die in the attempt. This will allow Pelias to remain king.
Jason gets really excited about the prospect of such an adventure, and many of the greatest heroes of Greece jump at the chance to participate: Hercules, Orpheus, Castor and Pollux, Achilles's father, and many more. Even Hera looks favorably on Jason.