Candide begins in the German town of Westphalia, where Candide, a young man, lives in the castle of Baron of Thunder-ten-tronckh. A noted philosopher, Doctor Pangloss, tutors the baron on philosophical optimism, the idea that "all is for the best . . . in this best of all worlds." Candide, a simple man, first accepts this philosophy, but as he experiences the horrors of war, poverty, the maliciousness of man, and the hypocrisy of the church, he begins to doubt the voracity of Pangloss's theory. Thus, philosophical optimism is the focus of Votaire's satire; anti-war and anti-church refrains also run throughout the novel.
In the first chapter, Doctor Pangloss is having an illicit affair with Paquette, a chambermaid. The baron's beautiful daughter, Cunégonde, witnesses the affair and decides to try something similar with Candide. When the baron catches them, Candide is kicked out of the castle.
Hungry and cold, Candide makes his way to a neighboring town, where he is aided by two soldiers. He is pressed into service and endures beatings at the hands of his superiors. He runs away, coming across war-torn villages in the process and witnessing the horrors of war firsthand. Candide makes his way to Christian Holland, where he hopes to find charity but finds hardhearted people, save one, an Anabaptist, who shows Candide kindness and generosity.
Candide then meets a beggar who is suffering from a disfiguring disease and soon discovers that the beggar is Doctor Pangloss. Pangloss recounts his recent experiences, including the death of the baron and his family at the hands of soldiers. In spite of Pangloss's condition and the horrors around him, the good doctor still believes in philosophical optimism. The Anabaptist sees to it that Pangloss is cured, and then takes him and Candide to Lisbon via ship. When a storm blows up, the Anabaptist is killed trying to save a sailor; the ship later breaks up, leaving Candide, Pangloss, and the rescued sailor as the only survivors. No sooner do they land on the Lisbon shore than an earthquake shakes the city; in response, church leaders decide to show an auto-da-fé, or act of faith, which includes a sacrifice of people. Pangloss is hanged, but Candide survives, helped by an old woman.
The old woman cleans and feeds Candide, and then takes him to Cunégonde, who survived the brutal attack on the baron's family. She is living with two powerful men who try to share her affections, and she was responsible for saving Candide from the killings during the auto-da-fé. Cunégonde's two men come upon the young lovers, and Candide kills them both. Frightened, Candide, Cunégonde, and the old woman escape to a port city, where a military vessel is loading up for a mission in Paraguay. Candide's military training impresses the Spanish general, and Candide is made a captain with command of an infantry. With Cunégonde and the old woman, Candide sails for South America. During the voyage, the old woman tells her story, which is horrific — she has suffered far more than anyone else in the party. Candide begins to seriously question Pangloss's theory of philosophical optimism.
In Buenos Aires, they meet the governor, Don Fernando, who takes an interest in Cunégonde and asks for her hand in marriage. Candide is heartbroken, but he cannot stay and fight for Cunégonde, because he must flee from police officers who traced Candide to the region. Aided by Cacambo, a valet, Candide escapes and soon meets the Reverend Father Commander, leader of a Jesuit army in Paraguay. The commander turns out to be Cunégonde's brother, who was left for dead when his mother and father were killed in Westphalia. The two catch up until Candide reveals that he is love with Cunégonde and hopes to someday marry her; the baron's son is so enraged by this notion that a fight ensues, and Candide kills the man.
Again, Candide flees with Cacambo and, before long, the two face the Oreillons, who at first nearly kill Candide but soon treat him hospitably. Upon leaving their company, Candide and Cacambo come to Eldorado, a country filled with gold and jewels for which the citizens have no use, because everyone's needs are met by the government. Eldorado also has no court rooms or prisons, because citizens treat each other fairly and do not break laws. The citizens of Eldorado believe in God but never pray in supplication — they only give thanks because they have all they need.
Eager to find Cunégonde, Candide and Cacambo leave Eldorado with a team of red sheep loaded with gold, jewels, and other supplies. When they reach Surinam, the two traveling companions split up, with Cacambo heading in secret to Buenos Aires to buy the release of Cunégonde, and Candide heading to Venice, where he will not be sought by the police. Candide is victimized by a ship's captain, a ruthless man named Mynheer Vanderdendur, and the judge from whom Candide seeks redress. Dejected, Candide advertises a contest for the most unfortunate man he can find; an elderly scholar named Martin wins the contest and becomes Candide's new traveling partner. The two head to France, en route to Venice.
In Paris, Candide becomes ill and is attended by a variety of people, all of whom want a piece of his fortune. He recovers, but is tricked by an actress into giving away much of his fortune and is eventually arrested by the police, who are suspicious of all strangers. From there, Candide and Martin are sent to England, where they witness more violence, and then finally reach Venice. Through various discussions and wagers with Martin, as well as meetings with a variety of people, Candide comes to lose faith in philosophical optimism. Soon, Candide finds Cacambo, now a slave, who informs Candide that Cunégonde is in Constantinople, working as a servant. Candide buys Cacambo's freedom, and the three men travel toward Constantinople. They soon meet Pangloss and the baron's son, both of whom were presumed dead, and discover that, back in Lisbon, the noose on Pangloss's neck slipped, while the baron's son recovered from Candide's stab wound. The five set off to find Cunégonde, who is with the old woman and is no longer beautiful, and Candide buys their freedom, as well. When the baron's son again steps in the bar Candide's marriage to Cunégonde (a marriage Candide no longer desires), the party kills the baron's son.
Candide marries Cunégonde and buys a small farm with the last of his Eldorado fortune. The entire party — Candide, Cunégonde, Cacambo, Martin, Pangloss, and the old woman — live there together, and are soon joined by Paquette and her companion, Friar Giroflée. They discuss philosophy and are utterly miserable until they meet a happy Turk relaxing under a tree. The Turk explains that he has only a small farm but he is happy because he works it with his children. The farm meets his needs and saves him from boredom and evil desires. Candide decides that this is how his little group will find happiness, and they begin to work their farm.