In Part 1, "Mosque," the novel opens with a panoramic view of the fictional city of Chandrapore, India. The narrative shifts to Dr. Aziz, who is called away from dinner with his friends by his superior at the hospital, Major Callendar. He then visits to the local mosque, where he meets Mrs. Moore, an Englishwoman who has recently arrived to visit her son, Ronny Heaslop, with his love interest, Adela Quested.
After the mosque, Mrs. Moore arrives at the club just as Adela announces to the members of the club that she would like to see the real India. To humor her, Mr. Turton, the Collector of the district, offers to set up a Bridge Party. At the Bridge Party, Adela is disappointed to find that the British and Indian guests are grouped in separate areas of the lawn. There, Adela meets Mr. Fielding, the principal of the local Government College, who invites her to a tea party where she might meet more Indians, including Mrs. Moore's friend Aziz.
At Fielding's tea party, Adela finally meets Aziz. The other guests include Mrs. Moore and Professor Godbole. While Fielding gives Mrs. Moore a tour of the grounds, Aziz and Adela continue their conversation. Aziz spontaneously offers to take Adela and the rest of the party for an excursion to the Marabar Caves. During this conversation, Aziz and Adela are interrupted by Ronny, who is angered to see Adela alone with an Indian. After Fielding's tea party, Adela tells Ronny that she is unwilling to marry him. Just then, the Nawab Bahadur offers to take them on a ride in his new car. They accept, but on the ride, get into a car accident. No one is hurt, and the accident is a bonding experience for Adela and Ronny, who agree to marry each other by the time they return home.
Aziz skips work for a few days feigning illness. Aziz's friends, including Hamidullah, stop by to see how he's doing, followed by Fielding. After the rest of the guests leave, Aziz detains Fielding in order to show him a photograph of his dead wife as a gesture of friendship.
In Part 2, "Caves," the hot season approaches. Upon hearing a false rumor that Adela is offended that Aziz has not invited them out to the caves, Aziz invites Adela and Mrs. Moore for an excursion to the caves, even though no one in the party wants to go.
On the morning of the picnic, Aziz meets Adela and Mrs. Moore at the train station, but Fielding and Godbole arrive too late to make the train. Aziz, Adela, Mrs. Moore, and the rest of their party proceed to the Marabar Caves. In the tour of the first cave, Mrs. Moore feels ill and is shaken to her spiritual core by the troubling echo within the cave. She stays behind at the picnic site while Adela follows Aziz and the guide to other caves. Just as Adela comes to a realization that she doesn't love Ronny, she offends Aziz, who slips into another cave to escape her. Distracted, Adela also walks into a cave.
Having recovered his composure, Aziz leaves the cave, and notices that Adela is missing. Aziz also notices that a car is driving by the hills below. When he returns to the picnic site, he is greeted by Fielding. Together with Mrs. Moore, they return to Chandrapore, but upon their arrival, Aziz is arrested for allegedly attacking Adela. Fielding spends the rest of the day working for Aziz's release. At the club, the British gather to discuss Adela's case. Fielding defends Aziz's innocence, in the process offending Ronny Heaslop and the rest of the club members. After Adela recovers from her injuries at the McBrydes' bungalow, she returns to Ronny's bungalow, where Mrs. Moore, still overwhelmed by her experience in the caves, speaks incoherently and unpleasantly to Adela. Unwilling to participate in the trial, Mrs. Moore, with Ronny's help, arranges to sail back to England.
After Mrs. Moore leaves, Adela stays with the Turtons. On the day of the trial, the Turtons take Adela to the courthouse. In the courtroom, McBryde opens the case against the defendant. When Adela takes the stand, she suddenly realizes her mistake and withdraws her charge against Aziz. The courtroom is thrown into a tumult. Adela is carried out of the courtroom by the crowds. Fielding rescues her and drives her back to the college for safety. Meanwhile, Aziz and his party decide to attack the hospital, but Lal's buffoonery defuses their riotous impulses.
Ronny visits Adela at Fielding's, where he notifies both of them of Mrs. Moore's death on the boat out to England. Fielding lets Adela stay at the college while he goes to Aziz's victory celebration, where he tries to convince Aziz not to pursue a lawsuit for damages against Adela. Later, Aziz decides not to press charges against Adela.
With Ronny breaking off their engagement, Adela returns to England. A rumor that Adela and Fielding had an affair while she was staying at the college strains Aziz's relationship with Fielding, who also leaves for England before their friendship is repaired.
In Part 3, "Temple," the novel fast-forwards several years to Mau during the rainy season. Godbole, the Minister of Education at Mau, directs the Gokul Ashtami festivities while Aziz, now a doctor at Mau, attends to the ailing ruler. On a walk to a local shrine, Aziz sees Fielding and a man get chased out by bees. The man turns out to be Ralph Moore, and Aziz realizes his mistake: Fielding has married Stella Moore, Mrs. Moore's daughter, not Adela. Aziz's antagonism toward Fielding and his party melts when he talks with Ralph alone in their guest quarters. Aziz takes Ralph out on the Mau tank to view the festivities, but their boat collides with Fielding and Stella's. Everyone ends up in the water. The novel ends as Fielding and Aziz go on a horse ride together, with the mutual realization that circumstances prevent them from maintaining their friendship.