Gloriana, Queene of Faerie Land, has appointed the Red Cross Knight to accompany Una on a journey to her kingdom to destroy a dragon that is ravaging the land and holding Una’s parents captive. A sudden shower forces Red Cross and Una into the woods and near a cave, where Red Cross defeats the monster Error. Continuing their journey, the pair meet Archimago (the “arch-magician”), disguised as a kind old man, who invites Red Cross and Una to spend the night at his home. In the night, Archimago summons demons and sends a dream to Red Cross of Una making sexual advances to him and, when this is unsuccessful, Archimago shows Red Cross the “False” Una in bed with another man.

Distraught, Red Cross leaves alone the next morning and soon meets the old witch Duessa, disguised as Fidessa, a young and beautiful maiden. Duessa is accompanied by Sansfoy, whom Red Cross kills in a fierce fight. Duessa and Red Cross then rest under a pair of trees. To Red Cross’s surprise, one of the trees begins to speak, describing how it was once a young knight named Fradubio who was traveling with his fair Fraelissa. Fradubio explains how he met a beautiful maiden, was enamored of her, and fought for her hand. The beautiful maiden then turned Fraelissa into a tree to end Fradubio’s love for Fraelissa, and later, after Fradubio saw his new love bathing and realized that she was actually an old and loathsome witch (Duessa), Fradubio himself is turned into a tree by the witch. Red Cross fails to understand the warning, and he and Duessa soon continue their journey.

In the meantime, Una, left behind by Red Cross, wanders in a forest, searching for her knight. She there meets a lion, who decides to protect the maiden. As night falls, Una and the lion seek shelter in the home of Abessa (who is dumb) and Corceca (who is blind); though they resist, the lion forces his way into their house. Later that night, Kirkrapine, a church robber and Abessa’s lover, enters the house, where he is killed by the lion. Una and the lion depart the next morning, only to be approached by Archimago, disguised as Red Cross. Una is fooled until the pair are stopped by Sansloy, who, seeing the red cross on Archimago’s chest, mistakenly takes him for Red Cross and challenges him to a duel to avenge the death of his brother, Sansfoy, earlier killed by the real Red Cross. Sansloy injures Archimago and then removes Archimago’s helmet, and both Sansloy and Una find that he is not Red Cross after all. The lion attacks Sansloy when Sansloy attempt to sexually assault Una, but Sansloy kills the lion and then forces Una onto his horse, and the two ride off into the forest.

The scene then shifts to Red Cross as he is led by Duessa into the House of Pride. Red Cross is impressed at first by the lush palace and soon witnesses a formal procession of the Seven Deadly Sins, with Queen Lucifera the focus of the procession. The entertainment is interrupted, though, by the arrival of Sansjoy, who, after seeing his dead brother’s (Sansfoy’s) shield in Red Cross’s possession, challenges the knight to a duel. Lucifera arranges a duel between the two men for the next day. The fight is treacherous, and both men are suffer great injuries. Just as Red Cross is about to kill Sansjoy, a dark cloud covers and protects Sansjoy. Red Cross is carried back to the House of Pride, where he is treated for his wounds. In the meantime, Duessa takes Sansjoy down to Hades (Avernus), where he is treated by the Greek physician Aesculapius. As Red Cross’s wounds are treated, the Dwarf warns him that he has seen the dungeon of the palace, and it is filled with victims of Pride and the other Deadly Sins. Thus warned, Red Cross, though weakened from his wounds, makes his escape from the House. Duessa, returning from Hades, discovers that Red Cross has departed.

Meanwhile, Sansloy attempts to seduce and to rape Una in the woods, but he is scared off by a group of fauns and satyrs. These creatures recognize Una’s beauty and take her to their leader, Sylvanus, to be worshipped as a goddess. Satyrane, a knight who is the son of a satyr father and a human mother, is visiting the woods when he meets Una, whom he helps to escape. On their way out of the woods, the pair meet a pilgrim who tells them that he witnessed the death of Red Cross at the hands of another knight, and the pilgrim informs the pair where this knight can be found. Satyrane discovers Sansloy and challenges him, while Una, recognizing Sansloy as her adversary, escapes from the scene, followed by the pilgrim, who is actually Archimago in another of his many disguises.

As Una makes her escape, Duessa begins searching for Red Cross and discovers him next to a magic fountain whose waters, once drank, cause a loss of strength. Duessa and Red Cross are reconciled, and, after drinking from the magic fountain and losing his powers, Red Cross “dallies” with Duessa on the grass until he hears the approach of Orgoglio, a hideous giant. Orgoglio quickly overcomes the weakened Red Cross, but Duessa asks that his life be spared, and, in return, she agrees to become the mistress of the giant. Red Cross survives but is thrown into Orgoglio’s dungeon. The Dwarf, after witnessing Red Cross’s defeat, sets out to find help and discovers Una, who is still fleeing from Sansloy. The Dwarf relates all of Red Cross’s adventures to Una, and the two then head toward Orgoglio’s castle. On their way, they meet a brilliantly arrayed knight, whose armor includes a magic diamond shield with such great powers that it can turn men to stone and overthrow monsters; the shield is so powerful, in fact, that it must remain covered with cloth. Una explains her situation to the knight, who comforts her and agrees to help. The knight, though unnamed, is the great Prince Arthur.

Arthur, accompanied by his Squire, Una, and the Dwarf, approaches the castle, and the Squire blows a horn whose blast bursts open the castle doors. Orgoglio comes running out, along with Duessa riding a seven-headed beast, a gift from Orgoglio. Arthur and his Squire engage in a great battle with Orgoglio and the beast and, in the midst of the fight, the cloth drops from Arthur’s magic shield, exposing its brilliant surface. Orgoglio and the beast are stunned by the light from the shield, thus allowing Arthur to kill both monsters. Duessa attempts to escape but is restrained by the Squire. After the battle, Arthur enters the castle in search of Red Cross, but he finds only an old servant named Ignaro who walks with his head facing backwards and who cannot answer any of Arthur’s questions. Arthur takes Ignaro’s keys and begins exploring the different rooms of the castle, finding one room richly decorated but containing an altar stained with the blood of martyrs. He opens another door and falls into a deep dungeon holding Red Cross, famished and distraught after months of imprisonment. After struggling out of the dungeon, Arthur returns Red Cross to Una, and the two reunite after their long absence. Duessa is allowed to live, but not until she is stripped of all her clothes, revealing her as a loathsome and foul hag.

Still unaware of the brave knight’s identity, Una and Red Cross question Arthur. Arthur explains that he does not know who his parents are: he was raised by Timon, an old knight, and educated by the magician Merlin, who would only tell him that Arthur’s father was a king and that Arthur would gain knowledge of his identity some time in the future. Arthur then describes how he was visited by the Faerie Queene in a dream, and, captivated by her beauty, he has been searching in vain for her in Faerie Land for the last nine months. He then leaves Una and Red Cross to resume his search. Una and Red Cross, likewise, resume their long-delayed journey but are soon interrupted by Sir Trevisan, a knight running along the roadside with a rope around his neck. Trevisan describes how the villain Despair attempted to persuade Trevisan and his companion, Sir Terwin, that their desperate lives should be ended by suicide. Terwin had stabbed himself, but Trevisan escaped just as he was about to hang himself. Red Cross vows to destroy Despair and, led by Trevisan, he enters the villain’s dark cave and confronts Despair, denouncing his persuasions. Despair, however, argues that his work should be praised, since he has helped so many people escape from the miseries of human existence, and he reminds Red Cross that even Red Cross himself has suffered his share of miseries and embarrassments. Despair is so convincing that Red Cross raises his dagger to end his own life, but Una intercedes, preventing the knight from taking his life, lecturing him on his foolishness, and reminding Red Cross of heavenly mercy. Convinced by Una, Red Cross escapes from the cave of Despair.

Seeing that the weakened Red Cross is in need of recuperation, Una leads him to the House of Holiness, where Red Cross is attended to and revitalized by a variety of characters, including Fidelia, Speranza, Patience, Penance, Remorse, and Repentance. Charity, another character residing in the House of Holiness, educates Red Cross on practicing love instead of hate, while Mercy instructs the knight on forms of charity. Contemplation leads the knight to the top of a high mountain and informs Red Cross that he will one day enter the New Jerusalem as St. George, the patron saint of England. Refreshed and restored, Red Cross once again rejoins Una on their journey to her native land.

Upon entering Una’s country, the pair see a huge dragon and a high tower that holds captive Una’s parents. Red Cross and the dragon immediately begin their fight, which lasts an entire day. Finally, Red Cross is able to injure the dragon, but, in return, the dragon breathes fire on Red Cross, burning him in his armor and causing him to fall into a spring. Believing he is victorious, the dragon rests as night falls. Una prays all night for the recovery of Red Cross, and, in the morning, Red Cross rises from the spring with his strength restored. Another day of fierce fighting follows, which again causes injury to both the dragon and to Red Cross. As the day ends, the wounded Red Cross falls at the foot of a blessed tree, whose stream of balm restores the knight for yet another day of fighting. On the third day, the dragon approaches Red Cross with open jaws, intending to eat the knight and to end the battle. Red Cross, though, pierces the throat of the dragon, finally killing the beast. Una steps forward to thank God and the brave knight for a great victory.

With the dragon killed, the land is freed from its captivity and, rejoicing, the inhabitants honor Red Cross as their hero. Ceremonies for the betrothal of Red Cross and Una are celebrated, until a messenger arrives with a letter stating that Red Cross is already pledged to Fidessa. Red Cross, supported by Una, denies the words of the messenger, and Una soon recognizes that the messenger is in reality Archimago in yet another disguise. Archimago is captured and thrown into a dungeon, and the betrothal of the knight and his lady is concluded. Red Cross, however, cannot remain with Una but must instead continue to fulfill his pledge of six years of service to Gloriana, the Queen of Faerie Land.