Acrisius, because he’s a jerk like nearly all other Greek monarchs faced with this kind of decision—and it’s does happen often (never go to Delphi!)—banishes his daughter and infant grandson. Okay, banish is not exactly the right term. He locks them in a box and shoves them out to sea.
Don’t worry, they make it to an island. There Perseus is raised by his mother and a fisherman. Unfortunately, King Polydectes has a crush on Danae and wants to marry her; she wants nothing to do with him. And so Perseus stands up.
We pause so I can give proper credit to one of my former mythology students (I haven’t obtained permission to use his name so I have to refer to him obliquely in this, but if I hear from him I’ll update with his name). Before this student, I had interpreted the myth in a completely different way (which I will go into as I tackle various parts of the myth). This student pointed out that Perseus’s defense of his mother is the central character trait. This is not only what kickstarts his quest, but what defines him as a hero. And this realization made everything fit into place for me.
So, Perseus objects to his mom being married to Polydectes. But he’s got nothing. He can’t bribe the king with a horse, so Polydectes names his own price, the head of Medusa. This is, largely, a fool’s mission, one that Polydectes does not expect Perseus survive, much less deliver on.
Undaunted, Perseus agrees. And this truly demonstrates just how much he cares about his mother, and his conviction that she should not have to be married against her will. We will see this trait reinforced later on. What used to just serve as the call to Perseus’s quest is the key to unlocking the secrets of his myth, and of other myths, but that comes later.