Just as Jason was being born, his evil uncle Pelias overthrew his father and became king. Jason’s mother hid him, and when he grew up, he came back and demanded his throne. Pelias agreed to give back the kingship, but first, he said, Jason should go on a quest to get the Golden Fleece, a sheepskin full of gold.
So Jason set out on a ship, the Argo, with some other adventurers, the Argonauts. After various adventures, they reached Colchis (on the other side of the Black Sea). There Medea, the king’s daughter, fell in love with Jason and agreed to help him get the Golden Fleece if he would take her with him when he left (like Ariadne and Theseus).
Medea gave Jason various magic tricks so he could overcome dragons and soldiers and get the Fleece. As they were running back to his ship, Medea kept her father from catching them by killing her brother and scattering bits of his body in her father’s path. (This is kind of like Atalanta‘s story but much creepier.) You can see this isn’t going to end well.