Book I tells the story of the knight of Holiness, the Redcrosse Knight. This hero gets his name from the blood-red cross emblazoned on his shield. He has been given a task by Gloriana, "that greatest Glorious Queen of Faerie lond," to fight a terrible dragon (I.i.3). He is traveling with a beautiful, innocent young lady and a dwarf as servant. Just as we join the three travelers, a storm breaks upon them and they rush to find cover in a nearby forest. When the skies clear, they find that they are lost, and they end up near a cave, which the lady recognizes as the den of Error. Ignoring her warnings, Redcrosse enters and is attacked by the terrible beast, Error, and her young. She wraps him up in her tail, but he eventually manages to strangle her and chops off her head. Error's young then drink her blood until they burst and die. Victorious, the knight and his companions set out again, looking for the right path. As night falls, they meet an old hermit who offers them lodging in his inn. As the travelers sleep, the hermit assumes his real identity--he is Archimago, the black sorcerer, and he conjures up two spirits to trouble Redcrosse.
One of the sprites obtains a false dream from Morpheus, the god of sleep; the other takes the shape of Una, the lady accompanying Redcrosse. These sprites go to the knight; one gives him the dream of love and lust. When Redcrosse wakes up in a passion, the other sprite (appearing to be Una) is lying beside him, offering a kiss. The knight, however, resists her temptations and returns to sleep. Archimago then tries a new deception; he puts the sprite disguised as Una in a bed and turns the other sprite into a young man, who lies with the false Una. Archimago then wakes Redcrosse and shows him the two lovers in bed. Redcrosse is furious that "Una" would spoil her virtue with another man, and so in the morning he leaves without her. When the real Una wakes, she sees her knight is gone, and in sorrow rides off to look for him. Archimago, enjoying the fruits of his scheme, now disguises himself as Redcrosse and follows after Una.
As Redcrosse wanders on, he approaches another knight--Sansfoy, who is traveling with his lady. He charges Redcrosse, and they fight fiercely, but the shield with the blood-red cross protects our hero; eventually, he kills Sansfoy. He takes the woman into his care--she calls herself Fidessa, saying that she is the daughter of the Emperor of the West. Redcrosse swears to protect her, attracted to her beauty. They continue together, but soon the sun becomes so hot that they must rest under the shade of some trees. Redcrosse breaks a branch off of one tree and is shocked when blood drips forth from it, and a voice cries out in pain. The tree speaks and tells its story. It was once a man, named Fradubio, who had a beautiful lady named Fraelissa--now the tree next to him. One day, Fradubio happened to defeat a knight and win his lady (just as Redcrosse did)--and that lady turned out to be Duessa, an evil witch. Duessa turned Fraelissa into a tree, so that she could have Fradubio for herself. But Fradubio saw the witch in her true, ugly form while she was bathing, and when he tried to run away, she turned him into a tree, as well. When Fradubio finishes his story, Fidessa faints--because she is, in fact, Duessa, and she fears that she will be found out. She recovers though, and Redcrosse does not make the connection, so they continue on their way.
Redcrosse is the hero of Book I, and in the beginning of Canto i, he is called the knight of Holinesse. He will go through great trials and fight fierce monsters throughout the Book, and this in itself is entertaining, as a story of a heroic "knight errant." However, the more important purpose of the Faerie Queene is its allegory, the meaning behind its characters and events. The story's setting, a fanciful "faerie land," only emphasizes how its allegory is meant for a land very close to home: Spenser's England. The title character, the Faerie Queene herself, is meant to represent Queen Elizabeth. Redcrosse represents the individual Christian, on the search for Holiness, who is armed with faith in Christ, the shield with the bloody cross. He is traveling with Una, whose name means "truth." For a Christian to be holy, he must have true faith, and so the plot of Book I mostly concerns the attempts of evildoers to separate Redcrosse from Una. Most of these villains are meant by Spenser to represent one thing in common: the Roman Catholic Church. The poet felt that, in the English Reformation, the people had defeated "false religion" (Catholicism) and embraced "true religion" (Protestantism/Anglicanism). Thus, Redcrosse must defeat villains who mimic the falsehood of the Roman Church.
The first of these is Error. When Redcrosse chokes the beast, Spenser writes, "Her vomit full of bookes and papers was (I.i.20)." These papers represent Roman Catholic propaganda that was put out in Spenser's time, against Queen Elizabeth and Anglicanism. The Christian (Redcrosse) may be able to defeat these obvious and disgusting errors, but before he is united to the truth he is still lost and can be easily deceived. This deceit is arranged by Archimago, whose name means "arch-image"--the Protestants accused the Catholics of idolatry because of their extensive use of images. The sorcerer is able, through deception and lust, to separate Redcrosse from Una--that is, to separate Holiness from Truth. Once separated, Holiness is susceptible to the opposite of truth, or falsehood. Redcrosse may able to defeat the strength of Sansfoy (literally "without faith" or "faithlessness") through his own native virtue, but he falls prey to the wiles of Falsehood herself--Duessa. Duessa also represents the Roman Church, both because she is "false faith," and because of her rich, purple and gold clothing, which, for Spenser, displays the greedy wealth and arrogant pomp of Rome. Much of the poet's imagery comes from a passage in the Book of Revelation, which describes the "whore of Babylon"--many Protestant readers took this Biblical passage to indicate the Catholic Church.
The Faerie Queene, however, also has many sources outside of the Bible. Spenser considers himself an epic poet in the classical tradition and so he borrows heavily from the great epics of antiquity: Homer's Iliad and Odyssey and Virgil's Aeneid. This is most evident at the opening of Book I, in which Spenser calls on one of the Muses to guide his poetry--Homer and Virgil established this form as the "proper" opening to an epic poem. The scene with the "human tree," in which a broken branch drips blood, likewise recalls a similar episode in the Aeneid. However, while these ancient poets mainly wrote to tell a story, we have already seen that Spenser has another purpose in mind. In the letter that introduces the Faerie Queene, he says that he followed Homer and Virgil and the Italian poets Ariosto and Tasso because they all have "ensampled a good governour and a vertuous man." Spenser intends to expand on this example by defining the characteristics of a good, virtuous, Christian man.