Britomart, a female knight, is the embodiment and champion of Chastity. She is young and beautiful, and falls in love with Artegal upon first seeing his face in her father's magic mirror. Though they have never met, she falls in love with him, and travels, dressed as a knight, accompanied by her nurse, Glauce, in search of her beloved Artegal. She carries an enchanted spear that allows her to defeat every Knight she encounters.

Guyon and Arthur meet Britomart, who wins a joust with Guyon. They separate as Arthur and Guyon leave to rescue Florimell, while Britomart rescues the Redcrosse Knight. Britomart reveals to the Redcrosse Knight that she is pursuing Sir Artegal because she is destined to marry him. They proceed to meet Merlin, who explains that Britomart's destiny is to found the English monarchy.

Britomart leaves and fights Sir Marinell while Arthur searches for Florimell. He is joined by Sir Satyrane and Britomart, and they witness and resist sexual temptation.

It is only at the end of her quest Britomart is challenged by two knights who both seek to avenge their previous defeats at the hand of the unknown Knight with the Ebony Spear, who is in reality Britomart. Both challenge, but again, are unseated by Britomart. In the heat of the challenge the face of Sir Artegal is revealed and Britomarts she at once recalls the day she first saw his face. Only then does her courage falter and her spirit grow tame, so that she softly withdraws her uplifted hand. When she hears the name of Artegal, her heart leaps and trembles with joy. She flushes deeply, and tries to hide her agitation by feigning anger. But all is put right and Britomart and Artegal fall deeply in love and cant bear to be parted. But Sir Artegal is on a quest, and parted they must be. So with a heavy heart Britomart joins Sir Scudamore in his search for Lady Amoret.