In the very beginning of the book, a group of suitors go to Penelope's house, so she decides to get Odysseus's bow. She announces that whoever can string the bow, and shoot an arrow through 12 axes, she will marry them.
All the suitors try to string and shoot the bow, but none of them succeed.
Odysseus, still in disguise as a beggar, takes Eumaeus, and Philoetius aside and tells them who he really is. He promises them that he will treat them like a son if they fight by his side against the suitors.