Pelias had seized the throne of Iolcus from his half brother Aeson, Jason's father, and when the adult Jason returned to the city to claim what was his by birth, an oracle had already warned Pelias to beware a man wearing only one sandal, which Jason happened to be doing after losing one crossing a river on his way into the city, helping an old woman who was in fact the goddess Hera in disguise. Rather than refuse Jason outright, Pelias set a task calculated to remove him permanently: bring back the Golden Fleece, the hide of a divine ram with golden wool, kept under guard by a sleepless dragon in the distant kingdom of Colchis.
To make the voyage possible, Jason had the shipwright Argus build the Argo, said to be the first long ship of its kind, and called together heroes from across Greece to crew it, a roster later tradition swelled to include figures such as Heracles, the twins Castor and Pollux, the musician Orpheus, and Atalanta, though which names belonged to the earliest versions of the story and which were added later is not settled among the ancient sources themselves. The voyage out took the Argonauts through a sequence of trials, most famously the Symplegades, a pair of rocks at the mouth of the Black Sea that clashed together to crush any ship attempting to pass, which the Argo cleared by sending a dove through first to gauge the timing and losing only the tip of its stern to the closing rocks.