Enduring Love is a postmodern novel that explores themes of narrative and knowledge through the relationships of the central character, Joe Rose. Joe is a science journalist in a "childless marriage of love" with Clarissa Rose, an English professor of Keats. Although Joe approaches the world through rationality and Clarissa approaches it through emotion, they are happy together and have a stable, comfortable world. Joe is the main narrator of the book and he often makes references to narrative choices of exclusion or inclusion of details. He explains that he chooses his beginning because it is the point that makes the most "sense." Joe starts the story with Clarissa and Joe's reunion picnic being interrupted by a hot air balloon accident. Joe and a few other men try to save a child trapped in a balloon, but the high winds force all the men except one to drop the ropes. That man, John Logan, a doctor and family-man from Oxford, is carried up into the sky by the balloon until he eventually falls to his death. His shocking and senseless death deeply upsets Joe and Clarissa and they try to comfort each other and make "sense" of the event by telling it as a story. Also at the balloon accident is a lonely, religious young man named Jed Parry. He shares a look with Joe and becomes convinced that they are in love. Later that night, as Clarissa and Joe are falling asleep, Parry calls Joe to tell him that he loves him.
Joe tries to cope with the disturbing accident by writing an article about narrative in science. While researching it, he thinks he sees Parry in the library and is unsettled. The next day, Joe tells Clarissa about Parry and how he is frightened, but Clarissa waves the situation off as a harmless crush. When Clarissa leaves for work, Parry calls Joe and asks to talk. Joe agrees to meet Parry, and Parry tries to talk to Joe about their love and how it is his mission to bring Joe to God's love. Joe feels confused and mildly threatened and leaves Parry, instead focusing on writing articles. Joe secretly feels dissatisfied with his successful journalism career, wishing he could instead be a real scientist. He was on the path to be an academic scientist when he got derailed by a failed patent application, leaving too large of a hole in his resume. Meanwhile, Parry calls Joe's house every few minutes, leaving thirty-three messages in total. The last message references "curtains," which intrigues Joe, but he cannot figure out why.
Clarissa comes home after having a bad day and does not want to deal with Joe's babblings about Parry. She becomes worried that Joe is going crazy when there aren't any messages on the machine, and their mutual distrust causes them to fight. Joe storms out of the house, where he sees Parry. Joe keeps walking, with Parry tailing him, shouting about his love. One of Parry's words, "signal," reminds Joe of "curtains," and he tries to figure out what the connection is. Parry writes a letter to Joe about how great their love is and how Parry will bring Joe to God. He tells Joe more about himself and asks Joe's forgiveness for not immediately recognizing the love between them when Joe did. After his fight with Clarissa, Joe grows paranoid that she is cheating on him and rummages through her desk. When he finds nothing, he grows disgusted with himself and drives to Oxford to visit John Logan's widow, Jean Logan. He realizes that his visit's purpose is to explain to both Jean and himself that he is not responsible for John Logan's death.
Jean Logan, a history professor at Oxford, is an emotional and physical mess when Joe arrives at her cold and plain house. She speaks harshly to him at first, telling him that she doesn't want any condolences. Instead, she wants Joe to help her with something. Jean found a strange woman's scarf and picnic basket in John's car and she now thinks he was having an affair. In an attempt to make sense of her husband's tragic death, she has constructed an explanation for his foolish actions and she wants Joe's help to prove it. While playing with her two children, Rachael and Leo, Joe reluctantly agrees to call the others involved in the accident to find out if they saw a woman with John. Watching the two children play in the curtains, Joe realizes that the curtains remind him of a famous case of de Clérambault’s syndrome, in which a French woman believed that the King of England was in love with her and sending her messages in the curtains. He races back to London to research more about the syndrome and Parry. This knowledge gives Joe power, since he is able to control Parry with words and narrative.
Joe finds Parry waiting outside Joe's apartment with a letter, upset that Joe's scientific articles try to disprove the existence of God. Joe ignores Parry and his threatening words and goes inside. He finds Clarissa in his study, upset that he has rifled through her desk. She thinks he is trying to send a signal with his actions, but she cannot understand it. Joe reads Parry's letter, in which Parry is very angry about Joe's resistance to God. As time passes and Parry sends more letters, Joe and Clarissa grow further apart. One day, while lying together on the bed, Clarissa tells Joe that she thinks their relationship is over. She's frightened that she's losing him, and she's frustrated that he keeps pushing her away. She's also secretly afraid that Joe is going crazy, so she decides to move into the guest "children's" room.
A few days later, Joe and Clarissa celebrate her birthday at a nice restaurant with her godfather, Professor Kale. While the three of them enjoy themselves by telling stories about DNA's discovery and the feud between Keats and Wordsworth, two men in masks enter the restaurant and shoot a man from the party sitting near Joe. A young man jumps up and prevents them from killing the man. Joe recognizes the young man as Parry and realizes that Parry has tried to have him killed. At the police station, Joe tries to convince the officers that he was the real target, but they don't believe him. He decides to take matters into his own hands and calls his old drug dealer, Johnny B. Well, to help him get a gun.
Joe and Johnny drive to the house of some ex-drug dealers who will sell him a gun. Johnny warns him not to laugh at them since they aren't too stable, but when Joe arrives there, his nervousness and the ridiculousness of the situation make him burst out in laughter. The three hippies, Steve, a man with a pointy, hennaed mustache, Xan, a muscular guy, and Daisy, a tired, middle-aged woman, are uneasy about selling Joe the gun, so Joe just puts the money down on the table. Xan and Steve physically fight over who gets it, while Daisy hands Joe the gun and tells them to leave quickly. On the way back to London, Joe gets a phone call from Parry, who says he's with Clarissa and that Joe should come back immediately.
After firing a few practice shots, Joe drives to his apartment, where Parry is holding Clarissa at knife-point. Joe and Clarissa try to talk Parry down, but Parry insists that he has something to ask Joe. Surprising both Joe and Clarissa, he asks Joe for forgiveness for trying to kill him. While Joe feels vindicated that he was right and Clarissa was wrong, he's still worried about what Parry will do. When Parry raises the knife, Clarissa flinches, but Parry instead brings it to his own neck and threatens to kill himself. Joe tries to talk him down but also shoots his elbow to disarm him. The police arrive soon after.
Clarissa writes a letter to Joe explaining her side of their argument in the fight they had after Parry's suicide attempt. While she admits that Joe was right, she argues that Parry becoming violent wasn't always inevitable, as Joe seems to think it was. She criticizes Joe's rational approach and explains that she's moving into her brother's flat, upset because she always thought that their love was the kind that lasted. A week later, Joe and Clarissa go to Jean Logan's house together to help her understand her husband's death. They go on a picnic to meet the woman whom Jean Logan thinks was in John's car the day of the accident. It turns out that Jean's narrative is wrong; John had picked up a hitchhiking couple on a picnic and was driving them when the accident happened. Jean feels guilty that she suspected her husband, and all members of the picnic reflect on how they need forgiveness.
Enduring Love has two appendices. The first is a case study of Jed Parry's de Clérambault’s syndrome. It explores the situation in scientific and psychiatric terms, while revealing that Parry is still in love with Joe but locked up, and that Clarissa and Joe have gotten back together and have adopted a child. The second appendix is a letter from Parry to Joe from the mental hospital he is staying in. The tone is joyous and light, and, even after one thousand days in the mental hospital, Parry is still in love with Joe. Joe never receives the letter, since the hospital staff keeps all Parry's letters to prevent disturbing Clarissa and Joe.