The Art of Racing in the Rain is narrated by the inner voice of a wise terrier-mix named Enzo, who lives in Seattle with his owner and personal hero, Denny Swift. The story begins as Enzo experiences a rapid decline in his health. Not only does Enzo know he is knocking on heaven's door, he embraces his death, because all Enzo's life, he has longed to be human; and ever since watching a T.V. documentary about reincarnation in Mongolia, Enzo has fully believed that when he dies, his soul will transfer to a newborn human. So, on the eve of his death, Enzo begins to look back on his life with Denny.
Denny works at an auto-body shop, but his true passion lies in racecar driving. He has a lot of talent for an amateur driver and years of professional training on racetracks. His specialty, as suggested by the title of the novel, is racing in the rain. From the time Denny picked Enzo out of a furry pile of puppies on a farm in Spangle, Washington, their favorite pastime is watching tapes of old races together. It is during this time together that Denny imparts his wisdom and philosophy about racing, resulting in Enzo growing up to be a racing fanatic.
But Denny and Enzo's bachelor lifestyle suddenly ends when Denny meets the love of his life, Eve. At first, Enzo feels threatened by Eve's relationship with Denny, but he quickly accepts her as an addition to their family when she becomes pregnant with their daughter, Zoë. Denny appoints Enzo as Zoë's protector, a responsibility which Enzo regards with the utmost seriousness for the rest of his life.
Enzo enjoys Zoë's infant days because there are always people in the house and playtime to be had. Life is good for a few years, but Enzo cannot help sensing a darkness growing in Eve's head. He suspects she might be sick, but his inability to communicate with humans prevents him from warning her.
As the years go by, Denny's racing career improves. He hustles for sponsorship money for the opportunity to race on the national stage, and eventually, he secures a seat in the Watkins Glen race with a prestigious team of racers. He wins first place overall, which earns him an offer in a touring car. Despite initial misgivings about being away from his family, Denny accepts the seat in the tour with Eve's encouragement.
But as her sickness progresses, the combination of Eve's mood swings and Denny's touring schedule as a semi-professional racer put a massive strain on their marriage. All the while, Eve refuses to see a doctor because she is scared of her diagnosis, and she feels fine for long enough stretches of time that she convinces herself she does not need to see a doctor.
During one of her long stretches of feeling in good health, Eve slips during a family hiking trip and hits her head on a rock. The serious fall leaves her with no other choice but to go to the hospital. At the hospital, doctors discover a mass in her brain and decide to operate immediately.
Eve's parents, Maxwell and Trish, to whom Enzo refers as "The Twins" because of their matching preppy clothes and snobbish demeanor, convince Denny to let Eve stay at their house on Mercer Island while she recovers from treatment. They insist it is the best option for Eve because they have more room and the resources to hire a round-the-clock nurse. Eve's parents never liked Denny; they find him unworthy of their daughter and look down on his blue-collar background. But Denny cannot deny that they have more time and resources to take care of Eve, so he agrees to have her stay with them.
As Eve's prognosis worsens, Maxwell and Trish become resigned to the idea that she will die soon, and suggest that Zoë also stay with them on Mercer Island so she can spend what precious time she has left with her mother. Little does Denny know, Maxwell and Trish are planning to keep Zoë and raise her themselves.
When it becomes clear to everyone that Eve's condition is terminal, Denny takes Zoë and Enzo to a family gathering of Eve's extended relatives in a cabin in the middle of Washington State. The gathering is a sort of family reunion over the long President's Day holiday. While staying in the cabin, Denny catches the eye of a 15-year-old girl named Annika. Annika takes every excuse to be alone with Denny and gratuitously dotes on Zoë and Enzo as a way of connecting with him.
When Denny decides to leave the cabin early to avoid an oncoming storm, Annika makes up an excuse as to why she also needs to head back to Seattle early and invites herself on Denny's road trip home. After an exhausting drive through a blizzard, Annika offers to stay at Denny's for the evening so he does not have to drive her home until morning. Denny agrees and sets up a cot for her. He puts Zoë to bed and collapses into his own bed.
While Denny is asleep, Annika takes a shower and then creeps into his room and disrobes. She takes Denny's pants off and fondles him while he sleeps. Enzo charges her, waking Denny up before things go any further. Denny understandably freaks out and tells Annika she needs to go home immediately. She calls her father to come pick her up.
Eve holds a party when she outlives her prognosis by one day, but dies later that night with Enzo by her side. Before she dies, Eve confesses to Enzo that she no longer fears death and accepts her fate. After Eve dies, Maxwell and Trish inform Denny that they are suing for custody of Zoe. Denny hires a good lawyer and it initially seems like The Twins stand no chance of winning custody, until the Seattle police show up at Denny's job and arrest him for third-degree rape of a child. Enzo suspects from the beginning that the whole situation was set into motion by Maxwell and Trish for leverage.
Denny is granted visitation rights with Zoe after the preliminary court date. Despite the limited time they have to see each other, their father-daughter relationship grows closer and stronger. The lawsuits go on for three years, and after a while, Denny completely runs out of money. He has to sell the house he bought for Eve and Zoë and move himself and Enzo into a shoebox apartment in downtown Seattle.
When Denny’s financial problems are at their most dire, Enzo gets hit by a car. Even though he can barely come up with the funds to cover Enzo’s veterinary bills, Denny still refuses to give up the fight for his daughter and turns down a settlement offer from Maxwell and Trish. Enzo helps make sure that Denny does not give up the fight by stealing the settlement papers just before Denny signs them and urinating on them. This gives Denny newfound confidence and motivation to continue the fight.
The tables turn for Denny when he is offered a job test-driving Ferraris on a well-known Italian racetrack. His spirits are lifted even further when he is told that the job can wait to begin until after his legal proceedings are resolved. Feeling a renewed sense of confidence and motivation, Denny is more determined than ever to win back his daughter’s custody and clear his name.
While walking one day, Denny and Enzo see Annika outside a coffee shop and confront her. He doesn’t confront Annika in an angry manner, but instead uses kindness and sympathy to try and reach her. He apologizes for if he ever sent her the wrong signals, and pleads with her not to take his daughter away from him. His speech brings tears to Annika and her friend's eyes.
Denny’s estranged parents visit him in order to meet Zoë. They only stay for a few days, and at the end of their visit, Denny's father gives him an envelope full of money. He took out a reverse mortgage on their family farm in order to give Denny money to keep his life afloat. Denny's father hopes that the gift will make up for their years of estrangement.
On the day of Denny's trial for the sexual assault charges, Annika recants her testimony, and the judge throws out the case. The very next day, The Twins drop their custody suit.
Throughout this time, Enzo has continued to age, but remained by Denny’s side to provide the help and support he needed. Once he feels that Denny is in a good place and no longer needs him, Enzo collapses in Denny’s arms and dies, praying that he is given the gift to return as a human.
The epilogue tells the reader of Denny’s success as a racecar driver and instructor in Italy. Zoë, now a grown woman, addresses the reader and describes a young boy named Enzo. She introduces Enzo as Denny's biggest fan. The conversation with human Enzo and his father causes Denny to recognize similarities between the boy and his beloved dog, and he offers to be human Enzo's racing instructor and mentor.