Candide contains thirty episodic chapters, which may be grouped into two main schemes: one consists of two divisions, separated by the protagonist's hiatus in El Dorado; the other consists of three parts, each defined by its geographical setting. By the former scheme, the first half of Candide constitutes the rising action and the last part the resolution. This view is supported by the strong theme of travel and quest, reminiscent of adventure and picaresque novels, which tend to employ such a dramatic structure. By the latter scheme, the thirty chapters may be grouped into three parts each comprising ten chapters and defined by locale: I–X are set in Europe, XI–XX are set in the Americas, and XXI–XXX are set in Europe and the Ottoman Empire. The plot summary that follows uses this second format and includes Voltaire's additions of 1761.
The tale of Candide begins in the castle of the Baron Thunder-ten-Tronckh in Westphalia, home to: the Baron's daughter, Lady Cunégonde; his bastard nephew, Candide; a tutor, Pangloss; a chambermaid, Paquette; and the rest of the Baron's family. The protagonist, Candide, is romantically attracted to Cunégonde. He is a child of "the most unaffected simplicity", whose face is "the index of his mind". Dr. Pangloss, professor of "métaphysico-théologo-cosmolonigologie" (English translation: "metaphysico-theologo-cosmonigology") and self-proclaimed optimist, teaches his pupils that they live in the "best of all possible worlds" and that "all is for the best".
All is well in the castle until Cunégonde sees Pangloss sexually engaged with Paquette in some bushes. Encouraged by this show of affection, Cunégonde drops her handkerchief next to Candide which entices him to kiss her. For this infraction, Candide is evicted from the castle, at which point he is captured by Bulgar (Prussian) recruiters and coerced into military service, where he is flogged, nearly executed, and forced to participate in a major battle between the Bulgars and the Abares (an allegory representing the Prussians and the French). Candide eventually escapes the army and makes his way to Holland where he is given aid by Jacques, an Anabaptist, who strengthens Candide's optimism. Soon after, Candide finds his master Pangloss, now a beggar with syphilis. Pangloss reveals he was infected with this disease by Paquette and shocks Candide by relating how Castle Thunder-ten-Tronckh was destroyed by Bulgars, and that Cunégonde and her whole family were killed. Pangloss is cured of his illness by Jacques, losing one eye and one ear in the process, and the three set sail to Lisbon.
In Lisbon's harbor, they are overtaken by a vicious storm which destroys the boat. Jacques attempts to save a sailor, and in the process is thrown overboard. The sailor makes no move to help the drowning Jacques, and Candide is in a state of despair until Pangloss explains to him that Lisbon harbor was created in order for Jacques to drown. Only Pangloss, Candide, and the "brutish sailor" who let Jacques drown survive the wreck and reach Lisbon, which is promptly hit by an earthquake, tsunami and fire that kill tens of thousands. The sailor leaves in order to loot the rubble while Candide, injured and begging for help, is lectured on the optimistic view of the situation by Pangloss.
The next day, Pangloss discusses his optimistic philosophy with a member of the Portuguese Inquisition, and he and Candide are arrested for heresy, set to be tortured and killed in an "auto-da-fé" set up to appease God and prevent another disaster. Candide is flogged and sees Pangloss hanged, but another earthquake intervenes and he escapes. He is approached by an old woman, who leads him to a house where Lady Cunégonde waits, alive. Candide is surprised: Pangloss had told him that Cunégonde had been raped and disemboweled. She had been, but Cunégonde points out that people survive such things. However, her rescuer sold her to a Jewish merchant who was then threatened by a corrupt Grand Inquisitor into sharing her. Her owners arrive, find her with another man, and Candide kills them both. Candide and the two women flee the city, heading to the Americas. Along the way, Cunégonde falls into self-pity, complaining of all the misfortunes that have befallen her. The old woman reciprocates by revealing her own tragic life, which included having a buttock cut off in order to feed some starving men.
The trio arrives in Buenos Aires, where Governor Don Fernando d'Ibarra y Figueroa y Mascarenes y Lampourdos y Alejandro asks to marry Cunégonde. Just then, an alcalde (a Spanish fortress commander) arrives, pursuing Candide for killing the Grand Inquisitor. Leaving the women behind, Candide flees to Paraguay with his practical and heretofore unmentioned manservant, Cacambo.
At a border post on the way to Paraguay, Cacambo and Candide speak to the commandant, who turns out to be Cunégonde's unnamed brother. He explains that after his family was slaughtered, the Jesuits' preparation for his burial revived him, and he has since joined the order. When Candide proclaims he intends to marry Cunégonde, her brother attacks him, and Candide stabs him through with his rapier. After lamenting all the people (mainly priests) he has killed, he and Cacambo flee. In their flight, Candide and Cacambo come across two naked women being chased and bitten by a pair of monkeys. Candide, seeking to protect the women, shoots and kills the monkeys, but is informed by Cacambo that the monkeys and women were probably lovers.
Cacambo and Candide are captured by Oreillons, or Orejones; members of the Inca nobility who widened the lobes of their ears, and are depicted here as the fictional inhabitants of the area. Mistaking Candide for a Jesuit by his robes, the Oreillons prepare to cook Candide and Cacambo; however, Cacambo convinces the Oreillons that Candide killed a Jesuit to procure the robe. Cacambo and Candide are released and travel for a month on foot and then down a river by canoe, living on fruits and berries.
After a few more adventures, Candide and Cacambo wander into El Dorado, a geographically isolated utopia where the streets are covered with precious stones, there exist no priests, and all of the king's jokes are funny. Candide and Cacambo stay a month in El Dorado, but Candide is still in pain without Cunégonde, and expresses to the king his wish to leave. The king points out that this is a foolish idea, but generously helps them do so. The pair continue their journey, now accompanied by one hundred red pack sheep carrying provisions and incredible sums of money, which they slowly lose or have stolen over the next few adventures.
Candide and Cacambo eventually reach Suriname, where they split up: Cacambo travels to Buenos Aires to retrieve Lady Cunégonde, while Candide prepares to travel to Europe to await the two. Candide's remaining sheep are stolen, and Candide is fined heavily by a Dutch magistrate for petulance over the theft. Before leaving Suriname, Candide feels in need of companionship, so he interviews a number of local men who have been through various ill-fortunes and settles on a man named Martin.
This companion, Martin, is a Manichaean scholar based on the real-life pessimist Pierre Bayle, who was a chief opponent of Leibniz. For the remainder of the voyage, Martin and Candide argue about philosophy, Martin painting the entire world as occupied by fools. Candide, however, remains an optimist at heart, since it is all he knows. As they arrive in England, they see an admiral (based on Admiral Byng) being shot for not killing enough of the enemy. Martin explains that Britain finds it necessary to shoot an admiral from time to time "pour l'encouragement des autres" (to encourage the others). Candide, horrified, arranges for them to leave Britain immediately. After various scenes satirising other European institutions, Candide and Martin meet Paquette, the chambermaid who infected Pangloss with his syphilis, in Venice. She is now a prostitute, and is spending her time with a monk, Brother Giroflée. Although both appear happy on the surface, they reveal their despair: Paquette has led a miserable existence as a sexual object, and the monk detests the religious order in which he was indoctrinated.
Later, while Candide and Martin are eating supper, Cacambo returns to Candide and informs him that Cunégonde is in Constantinople, and that she has been enslaved. She is now washing dishes for a prince of Transylvania, and has become ugly. On the way to rescue her, Candide finds Pangloss and Cunégonde's brother rowing in the galley. Candide buys their freedom and further passage at steep prices. The baron and Pangloss relate how they survived, but despite the horrors he has been through, Pangloss's optimism remains unshaken: "I still hold to my original opinions, because, after all, I'm a philosopher, and it wouldn't be proper for me to recant, since Leibniz cannot be wrong, and since pre-established harmony is the most beautiful thing in the world, along with the plenum and subtle matter."
The travellers arrive on the Ottoman coast where they rejoin Cunégonde and the old woman. Cunégonde has indeed become hideously ugly, but Candide nevertheless buys their freedom and marries Cunégonde to spite her brother. Paquette and Brother Giroflée, too, are reconciled with Candide on a farm which he just bought with the last of his finances.
One day, the protagonists seek out a dervish known as a great philosopher of the land. Pangloss asks him why Man is made to suffer so, and what they all ought to do. The dervish responds by asking rhetorically why Pangloss is concerned about the existence of evil and good. The dervish describes human beings as mice on a ship sent by a king to Egypt; their comfort does not matter to the king. The dervish then slams his door on the group. Returning to their farm, Candide, Pangloss, and Martin meet a Turk whose philosophy is to devote his life only to simple work and not concern himself with external affairs. He and his four children work a small farm to keep "free of three great evils: boredom, vice and necessity", or "poverty" as per John Butt's 1947 translation. Candide, Pangloss, Martin, Cunégonde, Paquette, Cacambo, the old woman, and Brother Giroflée all set to work (on this "louable dessein", or "commendable plan", as the narrator calls it) on a farm of their own, each to one specific task. Candide ignores Pangloss's insistence that all turned out for the best by necessity, instead telling him "we must cultivate our garden".