Book III concerns the virtue of Chastity, embodied in the knight Britomart. Canto i begins by praising Chastity, "That fairest vertue, farre above the rest (III.i.4)." The poem picks up where it left off at the end of Book II: following Sir Guyon (the hero of Book II) and Arthur. The two knights are searching for the Faerie Queene to offer their services to her. Riding across an open plain, they see another knight approaching, with his spear advanced. Sir Guyon charges but is knocked off his horse by the strange knight, who turns out to be the famous Britomart, a warrior damsel. Arthur makes peace between her and Guyon, and they ride on together. Suddenly, they see a young lady run past them, pursued by a forester who intends to rape her. Arthur and Guyon pursue them to save the lady, but Britomart continues onward; she has another quest in mind.

Approaching a castle, she sees in front a ring of six knights who are attacking a single valiant warrior. Britomart rushes to intervene, but the six knights say they have no choice. They are the servants of the lady of the castle, enforcing her decree: Whatever man passes by there, if he does not have a lady, he must serve the lady of the castle; if he does have a lady, he must admit that his love is less fair than the lady. The knight they attack is Redcrosse, who refused to profane the name of his love, Una. Then, Britomart challenges the decree and brings down four of the six knights before they accept defeat. The eight then enter Castle Joyeous. It is sumptuously decorated, with tapestries depicting the story of Venus (the goddess of love) and Adonis. Britomart presents herself before the lady of the cast, Malecasta, who does not realize that the knight is a woman--for Britomart refuses to take off her armor.

Malecasta, a lusty lady, is inflamed with passion for the knight, but Britomart misinterprets her affection as harmless friendship. When the castle is asleep, Malecasta sneaks over to Britomart's bed and lies down beside her; the warrior maiden wakes up in shock and, leaping form the bed, draws her sword. Malecasta seeing that her love is a woman, cries out and faints. Hearing her scream, the six knights of the castle and Redcrosse rush to the room, but Britomart fights them off again. She and Redcrosse feel they have overstayed their welcome and leave.

As they journey on, Redcrosse asks Britomart about her purpose in Faerie Land. She speaks in sorrow, saying that she is looking for a knight called Arthegall who has dishonored her. In fact, no dishonor has been done--she is in love with Arthegall but tempts Redcrosse to praise him by speaking ill of him. Britomart's father had a magic mirror given to him by Merlin; it could show to the viewer whatever he or she desired to see. By chance, Britomart had come across the mirror and, not yet knowing love, had asked to see the man she was destined to marry. She saw a handsome knight and was struck with love; soon, she could not sleep at night. She had never felt love before and was amazed at the hold it had on her. Her nurse, Glauce, finally learned from her what was the matter--she was afraid that she was in love with a shadow. She had no way of knowing if the mysterious knight even existed; if he did, where did he live? What was his name? Glauce tried to comfort her and used every kind of medicine and advice she could think of, but Britomart could not be consoled and began to waste away with the pangs of hopeless love.

As with Book I, Spenser begins Book III with a classical-style invocation of his Muse, Clio, and a humble criticism of his own poetry. However, in this book we will see how the poet is far more influenced by the Italian romantic epic than the classical epic. Homer and Virgil were extraordinary poets, but they were not most preoccupied with the subject of love; for this, Spenser finds Ariosto and Tasso much more useful. He imitates them in the character of Britomart, the warrior maiden; in the theme of battle fought to defend a maid's honor; and in the involvement of magical characters (like Merlin, whom we will see in the next Book). Of course, The Faerie Queene is also very different from the Italian romances; Spenser treats the trials of love with a high seriousness and makes it part of his ever-present allegory of Christian right and wrong. As a whole, the poem is more indebted to the Italian genre than anything else, but in the end its mood and the meaning under its surface are Spenser's own original creations.

Just as Redcrosse was (or became) the ideal personification of Holiness, Britomart is Chastity. She represents this by the purity of her love for Arthegall--which admits no lust--and by her resistance to those who would try to corrupt or dishonor true love, like the six knights and Malecasta. However, she also has other qualities, which show Spenser's view of chastity as a central and many-sided virtue. In modern times, we tend to see chastity simply as the avoidance of lust, but for Spenser it is something more positive. Britomart is strong in battle, which reflects the strength of will that chastity gives a person; in fact, her strength saves Redcrosse, which proves that chastity is essential to holiness. Outside of battle, though, she is weak and humble, showing the Christ-like sides of chastity. Of course, Britomart also shows some weakness in these first two cantos, when she is nearly ruined by the love of the strange knight in Merlin's mirror. This is due to her inexperience; just like Redcrosse, she is in some need of maturing.

Another similarity between Book I and Book III is the use of a House (castle) to represent a particular virtue or vice or a group of several. Here in Canto ii, we have the House of Joyeous (joyfulness), which does not seem like anything bad or immoral. We see, though, that the place has a most un-Christian joy: the joy of carelessness and the indulging of pleasures. Malecasta, appropriate to her name--which literally means "badly chaste"--is the opposite of Britomart, just as Duessa was the opposite of Una. Her "love" is nothing but physical desire; mistaking Britomart for a man, "her fickle hart conceived hasty fire...she was given all to fleshly lust, / And poured forth in sensuall delight (III.i.47-48)." Spenser makes fun of Malecasta's "fickle hart" by having her accidentally fall for another man--she is so fast, she doesn't even wait for a knight to get out of his (or her) armor.

It is a sign of Britomart's innocence that she does not immediately see Malecasta's desire for what it truly is. Likewise, her vision is clouded by the sight of Arthegall in her father's mirror; rather than rejoicing that she will have such a fine husband, she frets over the new feeling in her heart. She misinterprets it "Yet [she] thought it was not love, but some melancholy (III.ii.27)." Glauce, her nurse, tries to comfort her, saying, "For who with reason can you aye reprove, / To love the semblant pleasing most your mind, / And yield your heart, whence ye cannot remove (III.ii.40)." That is, love is in accord with reason, is not tainted by lust, and is fated anyway, so why resist it? Britomart resists because she cannot admit that any feeling so strong can still permit chastity; this negative view of the virtue is what she must change in the course of the Book.