Alonso Quixana is an older gentleman who lives in La Mancha, in the Spanish countryside. He has read many of the books of chivalry and as a result, he has lost his wits, and he decides to roam the country as a knight-errant named Don Quixote de La Mancha. Neither his niece nor his housekeeper can stop him from riding his old horse, Rocinante, out into the country. Quixote's first sally ends quickly. He insists on having an innkeeper knight him into the chivalric order. Quixote believes that the inn is a castle. Returning home for clothes and money, Quixote is beaten and left for dead. A commoner rescues Quixote and brings him home.
The niece and housekeeper deliberate with two of Quixote's friends, the priest and barber, and they decide to destroy Quixote's library, burning many of the books of chivalry. These books are the culprit. When Quixote recovers, he asks for his books and his niece tells him that the sage Muñaton has taken them. Quixote believes it was the sage Friston, his mortal foe. Having found a squire, a common peasant named Sancho Panza, Quixote leaves yet again. This second sally provides the story for the rest of Book I. Panza quickly realizes that his master is mad, but the squire hopes that Quixote will make good on his promise to name Sancho as the Governor of an island. Quixote attacks a windmill, believing it to be a giant, destroying his lance in the process. Indeed, Quixote gets involved in several altercations and violent disputes while traveling on the road.
There is a peaceful and pastoral interlude when Quixote joins the goatherds who mourn the death of their friend Chrysostom, a poet who died of a broken heart. Continuing on the road with Sancho, Quixote has a run in with some horse-breeders and he is beaten so badly that Sancho has to quickly get the knight to an inn. Quixote perceives the inn to be a castle, yet again. Quixote believes the innkeeper's daughter to be a beautiful princess who has promised to come to his bed during the knight. Later that night, Quixote ends up caressing Maritornes: the half-blind, hunchbacked servant girl. Her lover, a mule carrier, is enraged and the carrier beats Quixote when he realizes that his lover, Maritornes, is struggling to get away from Quixote. In the darkness a brawl ensues, including Sancho, Maritornes, the innkeeper, the mule carrier and Quixotewho quickly passes out. An officer of the Holy Brotherhood enters the room, having heard the commotion, and he fears that Quixote is dead.
Quixote is not dead. When he revives, he asks for the ingredients so that he might prepare for himself the "true balsam of Fierabras." He prepares the balsam, vomits, passes out, and wakes up feeling better. Sancho drinks the balsam and nearly dies. The next day, knight and squire leave the inn without paying. Quixote believes it to be an enchanted castle and he is offended by the suggestion that he should pay. Sancho does not escape as easily as Quixote does. Indeed, the squire is tossed in a blanket and his bags are stolen. In an arc of violence, Quixote murders some sheep, loses some teeth, steals a barber's basin (believing it to be Mambrino's helmet) and sets free a chain of galley-slaves who repay the knight's kindness with bruises.
Quixote befriends Cardenio, The Ragged Knight of the Sorry Countenance, who mourns the fact that his true love, Lucinda, has married another man: Don Fernando. Cardenio has gone mad with grief, running half-naked through the hills of Sierra Morena. Quixote imitates Cardenio, pining for his beloved lady, Dulcinea del Toboso. Quixote sends Sancho with a letter to deliver to Dulcinea but instead Sancho finds the barber and priest and leads them to Quixote.
With the help of Dorotea, a woman who has been deceived by Don Fernando, the priest and barber make plans to trick Don Quixote into coming home. Dorotea pretends to be the Princess Micomicona, desperately in need of Quixote's assistance. The final chapters of the novel combine romantic intrigue with the comedy of errors surrounding Don Quixote. Dorotea is reunited with Don Fernando and Cardenio is reunited with Lucinda. This takes place at the same inn which Quixote visited earlier (where was boxed by Maritornes' lover). Numerous guests arrive at the inn, as long-lost brothers are reunited, two other pairs of lovers are blessed and Don Quixote is almost arrested. The Holy Brotherhood has an arrest for Quixote's arrest on account of his "setting at liberty" a "group of galley-slaves." The priest begs for the officer to have mercy on Quixote because the knight is insane. The officer assents; Quixote is locked in a cage and carted home. Quixote believes the cage to be an enchantment, but when it is clear that he is going home he does not fight back. Of course, in Book II, Quixote goes out on his third and final sally, so Book I is not resolved.