Enzo, an elderly dog, is sprawled on the kitchen floor of his owner, Denny's, apartment in a puddle of his own urine. He tells the reader that he's staging this display so that Denny, who has been through so much in the last few years, will see that it's time to let Enzo go. When Denny gets home he gives Enzo a bath, cleans up the mess, and calls Mike, his friend and coworker, and asks him to cover for him the next day so he can take Enzo to the vet. He says he's not sure it's a round trip visit, and despite having set it up, Enzo is surprised. He reaffirms though that it's for the best, and now Denny can be free.
Enzo loves racing, and sprinkles racing strategy, wisdom, and stories throughout the novel. His favorite driver is Ayrton Senna, a charismatic driver who drove exceptionally well in the rain, just like Denny does. The events surrounding his death remain a mystery.
Enzo goes back in time to explain the ten years of events leading up to his “display.” Denny, a professional racecar driver, adopted him as a puppy and moved him to Seattle. For the first year it was just Denny and Enzo, but then Denny met Eve and quickly fell in love with her. Enzo tried to love her, but resented her for coming between himself and Denny. Denny and Eve were married within the year and Eve soon became pregnant. When the baby was born, Denny was across the country competing. After the birth, Eve asked Enzo to protect her daughter, whom they named Zoë. Denny returned the next day and shares that another driver on his team crashed their car and he never even got to drive.
The next several months passed quickly and happily until Eve and Denny went back to work and Zoë was put in daycare. Enzo was left home alone. He was bored and lonely until one day Denny left the TV on by accident, and Enzo spent the entire day watching. From that time on, Denny leaves the TV on for Enzo during the day, and Enzo's education truly begins. Enzo believes himself to have a very human soul, and uses television to learn how to be more human. After seeing a documentary on Mongolia, he learns that dogs are reincarnated as men when they die, and this becomes his goal. He spends the rest of his life trying to be as human as possible to prepare for his next life as a human.
After Zoë's second birthday, the family moves into a small house. Enzo can smell something wrong with Eve, although she doesn't know it yet. She begins experiencing sporadic episodes of migraines, nausea, and mood swings. One weekend, when Denny is gone for a race, Eve experiences a headache so bad she packs up Zoë and leaves for her parents' house, leaving Enzo alone for three days. Enzo rations the toilet water but can do nothing about food, and on the second night he begins to hallucinate. He sees Zoë's favorite toy, a stuffed zebra, come to life and molest all of her other toys. When Enzo goes to attack, the zebra eviscerates itself. When Denny returns to find Zoë's toys in ruin, he hits Enzo. Enzo believes the zebra framed him.
The following year, Denny secures a seat in a traveling racecar for a season. It means many absences, but Eve encourages him to go. The first few races go very poorly for Denny, and when he and Eve are discussing it at dinner one night, Denny says he needs to go away the following week to practice with his crew. Eve is angry and scared, and Zoë is refusing to eat her dinner, leading to a bigger fight. Eve finally agrees to make Zoë a hot dog, but when she tries to cut open the package, the knife slices into her hand. Eve, terrified, refuses to go to the doctor and Denny agrees to bandage it at home.
The season improves for Denny, and Eve's health improves for no apparent reason. In August, the family goes to the Slippery Slabs, a spot on a creek where Zoë can play. While lifting Zoë, Eve slips and falls on the rocks, hitting her head hard. Denny rushes her to the emergency room where they discover a large mass in her brain. Eve spends months in the hospital. Trish and Maxwell, Eve's parents, talk Denny into having Eve stay at their house when she's released, and to allow Zoë to stay with them as well so she can spend as much time as possible with her dying mother. Denny begrudgingly agrees. On her first night home, Eve, terrified, asks Enzo to protect her and not let her die that night. He stays awake the entire night.
Several months pass. In February, Denny, Enzo, and Zoë go to the mountains with Eve's extended family so Zoë can meet them. While there, a teenage daughter of one of Eve's cousins, Annika, develops a crush on Denny. When she learns that Denny will be leaving early to beat predicted bad weather, she decides she needs to leave early as well, and Denny agrees to take her. The five-hour drive takes ten due to the weather, and Annika decides to stay with Denny that night. Denny and Enzo fall asleep, and Enzo awakes to see Annika at the foot of Denny's bed taking Denny's pants off, and Enzo tells the reader that what she did must have been without Denny's consent. Finally, Enzo barks and wakes Denny, who leaps away, horrified. Annika tells Denny she loves him, but he refuses to engage with her. She calls her father and he comes to pick her up.
In the spring, Denny takes Enzo to California with him to a racetrack where he'll be driving for a television commercial, and he takes Enzo out on the track for a speed lap. Enzo loves the experience and it cements his love of racing. A month after they return to Seattle, Eve dies. Denny gets the phone call while he's at the dog park with Enzo, and overcome by emotion, Enzo runs away and kills and eats a squirrel. When Denny finds him later, they drive to Maxwell and Trish's house so Denny can say goodbye. After he does, Maxwell and Trish tell Denny that they're suing him for Zoë's custody.
Denny hires Mark Fein, a lawyer whose car Denny works on at the auto shop. He tells Denny that the suit is bogus and it'll be an easy win. Later that day, however, police officers come to Denny's work to arrest him for felony rape of a child—Annika's family had decided to press charges for what happened in February. Mark pays Denny's bail, and Denny and Enzo attend Eve's funeral a few days later. Enzo is diagnosed with hip dysplasia after experiencing major hip pain from walking hours to and from the funeral.
As winter arrives, Seattle gets a light dusting of snow. On a walk one night, Enzo is hit by a car. When Denny tries to pay the vet, he discovers he has no money, and Denny is embarrassed and ready to give up. A few weeks later, Denny and Enzo go to visit Mike to sign a settlement granting Denny a generous visitation schedule and settling for non-felony charges regarding Annika's case. Mike hands Denny a souvenir pen from the zoo to sign with, and Enzo sees a zebra floating in the pen. He realizes that the zebra isn't an outside demon, but rather a force within all of us, and he decides that Denny isn't going to accept the settlement. Ignoring the pain from his hips, Enzo grabs the papers off the table and leads Mike and Denny on a chase through the house, culminating in a leap out the window. In the backyard, Enzo urinates on the papers, and Denny decides he doesn't want to give up.
Later that summer while Denny is teaching at the Seattle racing school, Luca Pantoni, a man who works for Ferrari, asks Denny to show him around the track. After Denny lays down some hot laps, wowing the students and Enzo, Luca offers Denny a job testing cars and teaching for Ferrari in Italy. Denny declines, saying he can't leave the state, and Luca says the job will stay open until Denny is ready.
One winter evening when Denny and Enzo are out for a walk, they spot Annika sitting at an outdoor cafe. When they reach Annika, both Denny and Annika feign surprise at their meeting and Denny asks if he can sit down and speak with her for a moment. He apologizes for what happened and tells Annika that a relationship between them would never have worked. He says that the first time he saw Eve he could barely function, and he hopes that Annika finds someone someday that makes her feel like that. Finally, he says that because of her suit, he'll never be allowed to see Zoë again. When Denny is finished he and Enzo trot home, triumphant.
Denny's parents, whom Enzo has never met, come to visit. Denny's mother is blind and when she meets Zoë, Zoë sits very still while her grandmother explores her face. On the final night of their visit, Denny's father explains to Denny that they took out a reverse mortgage on their house so Denny could pay his legal fees. When Mike asks the next day, Enzo learns that Denny's parents effectively disowned him when he refused to care for his mother, but he had slowly built up a relationship over the last several years.
Denny's criminal trial begins soon after. Every day, Mike escorts Denny to court while Tony, Mike's partner, takes care of Enzo. On the third day, Tony receives a phone call that something is happening, and he and Enzo rush to the courthouse. They wait in the rain, and Enzo falls asleep and dreams of testifying in court using Stephen Hawking's voice synthesizer. He wakes to hear Denny saying that it's over, he won. Trish and Maxwell drop their custody suit the next day.
While Denny is making cookies in preparation for Zoë's return, the phone rings and it's Luca Pantoni. Denny says he'd like to accept Luca's offer, and asks Luca why he's made such a generous offer. Luca says that his own wife died, and it was the help from a mentor, his predecessor at Ferrari, that saved him, and so he wished to pass the gift on.
The next day, Enzo can barely get up. He goes to the kitchen where Denny is making pancakes and collapses. Denny cradles him, and Enzo experiences visions of the fields where he was born and flashes of the documentary on Mongolia. He starts to run through the fields, still hearing Denny's voice, and dies in Denny's arms.
The text jumps to a point in the future, where Denny has just won a Formula One race on the same track where Senna died. Zoë, now an adult, pulls up in a golf cart with two of Denny's fans, a father and a son. They ask for Denny's autograph, and Denny asks the boy his name. The boy replies that his name is Enzo, and he's going to be a champion. Denny gives the father his phone number and offers to teach Enzo to drive when he's old enough.