The story is told as a series of memories by Jacob Jankowski, a 93-year-old man who lives in a nursing home. Jacob is told what to eat and what to do.
As the memories begin, Jacob is a 23-year-old Polish American preparing for his final exams as a Cornell University veterinary student when he receives the devastating news that his parents have been involved in a car accident. Jacob’s father was a veterinarian and Jacob had planned to join his practice. When Jacob learns that his parents have been killed in the car crash and their home has been mortgaged to pay for his tuition and is not to become his, he has a breakdown and leaves his Ivy League school just short of graduation. In the dark of night, he jumps on a train, a circus train belonging to the Benzini Brothers Most Spectacular Show on Earth. On the train he is befriended by Camel, an old man and circus veteran, who persuades his companions not to throw Jacob off the train and takes him under his wing, findinghim odd menial jobs. When the owner of the circus, Uncle Al, learns of his training as a vet, he is hired to care for the circus animals. This leads Jacob to share quarters with a dwarf named Walter (who is known as Kinko to the circus) and his dog Queenie. A few weeks later Jacob is summoned to take a look at Camel, who, after drinking "Jake" (adulterated Jamaican ginger extract) for many years, can't move his arms or legs. Fearing Camel will be "red-lighted" (thrown off a moving train as punishment or as severance from the circus to avoid paying wages), Jacob hides him in his room.
The equestrian director, August, is a brutal man who mistreats the animals in his care (such as the new elephant, Rosie) and the people around him, but he can also be charming and generous. Jacob develops a guarded relationship with August and his wife, Marlena, with whom Jacob falls in love. August is suspicious of their relationship and beats Marlena and Jacob. Marlena subsequently leaves August and stays at a hotel while she is not performing. Uncle Al then informs Jacob that August is a paranoid schizophrenic and utters a threat: reunite August and Marlena as a happily married couple or Walter and Camel get red-lighted.
A few days later after discovering that August has tried to see Marlena, Jacob visits her in her hotel room. Soon after he comforts her, they sleep together, and soon declare their love for each other. Marlena soon returns to the circus to perform (and have secret meetings with Jacob), but refuses to have August near her, which makes Uncle Al furious. She also discovers that she is pregnant.
One night Jacob climbs up and jumps each car, while the train is moving, to August's room, carrying a knife between his teeth intending to kill August. However, he backs out, leaving the knife on August's pillow, and returns to his car, only to find no one there but Queenie. He then realizes that Walter and Camel were red-lighted, and he was also supposed to have been.
As the story climaxes, several circus workers who were red-lighted come back and release the animals, causing a stampede during the performance.
In the ensuing panic, Rosie takes a stake and drives it into August's head. His body is then trampled in the stampede. Jacob was the only one who saw what truly happened to August. As a result of this incident the circus is shut down. Soon after, Uncle Al's corpse is found with a makeshift garrote around his neck. Marlena and Jacob leave, taking with them circus animals including Rosie, Queenie and Marlena's horses, and begin their life together by joining the Ringling Bros circus, and later Jacob becomes the chief veterinarian at the Brookfield Zoo in Chicago where they settled.
Back in the nursing home Jacob is waiting for one of his children to take him to the circus. It is revealed that Jacob and Marlena married and had five children, spending the first seven years at Ringling before Jacob got a job as a vet for the Chicago zoo. Marlena is revealed to have died a few years before Jacob was put into the nursing home. After finding out no-one is coming for him, Jacob makes his way to the circus next to the nursing home on his own. He meets the manager Charlie and after the show begs to be allowed to stay with the circus selling tickets, Charlie agrees and Jacob believes he has finally come home.