After the death of King Cretheus, the Aeolian Pelias usurped the throne from his half-brother Aeson and became king of Iolcus in Thessaly (near the modern city of Volos). Because of this unlawful act, an oracle warned him that a descendant of Aeolus would seek revenge. Pelias put to death every prominent descendant of Aeolus he could, but spared Aeson because of the pleas of their mother Tyro. Instead, Pelias kept Aeson prisoner and forced him to renounce his inheritance. Aeson married Alcimede, who bore him a son named Jason. Pelias intended to kill the baby at once, but Alcimede summoned her kinswomen to weep over him as if he were stillborn. She faked a burial and smuggled the baby to Mount Pelion. He was raised by the centaur Chiron, the trainer of heroes.
When Jason was 20 years old, an oracle ordered him to dress as a Magnesian and head to the Iolcan court. While traveling Jason lost his sandal crossing the muddy Anavros river while helping an old woman (Hera in disguise). The goddess was angry with King Pelias for killing his stepmother Sidero after she had sought refuge in Hera's temple.
Another oracle warned Pelias to be on his guard against a man with one shoe. Pelias was presiding over a sacrifice to Poseidon with several neighboring kings in attendance. Among the crowd stood a tall youth in leopard skin with only one sandal. Pelias recognized that Jason was his nephew. He could not kill him because prominent kings of the Aeolian family were present. Instead, he asked Jason: "What would you do if an oracle announced that one of your fellow-citizens were destined to kill you?" Jason replied that he would send him to go and fetch the Golden Fleece, not knowing that Hera had put those words in his mouth.
Jason learned later that Pelias was being haunted by the ghost of Phrixus. Phrixus had fled from Orchomenus riding on a divine ram to avoid being sacrificed and took refuge in Colchis where he was later denied proper burial. According to an oracle, Iolcus would never prosper unless his ghost was taken back in a ship, together with the golden ram's fleece. This fleece now hung from a tree in the grove of the Colchian Ares, guarded night and day by a dragon that never slept. Pelias swore before Zeus that he would give up the throne at Jason's return while expecting that Jason's attempt to steal the Golden Fleece would be a fatal enterprise. However, Hera acted in Jason's favour during the perilous journey.