Poor Redcrosse. It's hard being a virtuous man in a tough world. Good thing the lovely Una and the brave Arthur are out searching for him.
Soon, they come to the giant's castle and Arthur takes out a magical horn that once blown, opens every door in the castle.
The giant, perturbed by this noise, comes out and begins to fight with Arthur, who proves more than his match, and the giant finds himself sprawled out on the ground in no time.
Duessa, horrified by what she sees, rushes into the fight, but is stopped by Arthur's young squire.
In return, Duessa casts a spell on the squire that causes him to lose his strength and sends her pet seven-headed beast to attack him.
When Arthur sees this, he rushes to save his squire, and severely injures the seven-headed beast. However, the giant has meanwhile revived himself and attacks Arthur. The giant is even more vicious and angry than before.
Just before he strikes, however, the veil falls from his shield and the most incredible, beautiful, and powerful light bursts forth that blinds the beast and knocks all the strength out of the giant.
Seeing this, Arthur cuts off the giant's leg and kills him. Duessa then tries to flee, but the squire prevents her.
Una, relieved at the outcome of the fight, thanks both Arthur and his squire for helping her and promises to help them in return. She urges them not to let Duessa go, but first, to find poor Redcrosse.
Arthur immediately goes in search of him and comes across a strange old man named Ignaro, who is blind and always looking behind him instead of forward. He's the father of the giant and is now the keeper of the keys to the castle.
Arthur asks him over and over again where the prisoners are, but the old man never knows the answer and finally Arthur just grabs the keys for himself and begins opening various doors.
He finds many rooms filled with treasure and with the blood of sacrilegious sacrifices, but no sign of Redcrosse.
Finally, he comes to an iron door that none of the keys open and he calls out and hears someone inside respond piteously, explaining he's been a prisoner for a long time.
Hearing this, Arthur bursts open the door and enters into the cell. Even though the cell turns out to be a deep, dark pit, Arthur manages to get him out.
Poor Redcrosse can barely tolerate the pain of the sun, since he's so unused to it, and he looks terribly pale and thin.
Una can barely contain her delight at seeing him, though she feels sad that he is in such bad shape. She wails about all the bad things that have happened to him.
Arthur tells her not to worry, that they can learn from these bad experiences, and that they need to decide what to do with Duessa.
Una says killing her would be too vindictive, but that they should take her magical robes and send her away.
They do that, plus some, and strip her completely naked so that they can see her "misshaped parts" (I.vii.46)—not the nicest bunch, here—and that even though she tricks men into believing she's beautiful, she is actually old, ugly and monstrous.
Una points out that now they are seeing the true Duessa and the true nature of lying.
Duessa flees to hide herself from the world while the knights, Una, the dwarf, and the squire stay in the giant's castle to rest.