NOTE: All citations in this Study Guide refer to the Harper Collins version of Mercury, published in 2016.

Viv wants to be a championship winning horsewoman. Viv destroys her own life, damages her family, and badly injures a close friend in an attempt to pursue this dream. When Donald, Viv's husband, scrutinizes of the events that lead to these tragedies, Donald realizes he is perhaps as much at fault for Viv’s actions as she is.

After the death of his father, Donald is distracted by his grief and distant from his family. He is angry with Viv for spending more and more time away from the family taking care of a new horse, named Mercury, at the stables where she works. When she finances an alarm system for the barn with her own credit card, Donald is angry with Viv, especially since it is after this purchase that Viv stops pushing the idea that their son attend private school. Even though Donald does not want Marcus in private school, he believes Viv’s change of mind is a sign that she cares more for the horse than she does for her family. In an attempt to get revenge on Viv, Donald chooses not to tell her that he has been at the stables late one evening and found that Charlie, one of the stable girls, has been breaking into the barn to ride Mercury.

Viv’s side of the story comes next. She admits that as long as she could remember, she has wanted to be a championship equestrian. In college she over-trained a horse resulting in an injury that not only knocked them out of the three-day competition she was on track to win, but also resulted in the horse’s death. After that experience, Viv tries to gratify her ambitious nature in the business world. When she meets Mercury, she thinks that her life’s purpose is to win championships with him.

As Viv trains on Mercury, she becomes more and more paranoid that someone is trying to hurt him. Viv believes the burglars who break into the barn were doing so because they are interested in Mercury. The break-ins continue despite increased security and Viv buys a gun. One night, she returns to the stables believing she has forgotten to set the alarm. She encounters three people in Mercury’s stall. In her desire to protect the horse, she aims the gun intending to shout out and scare the people away. Instead her finger tighten on the trigger and she shoots one of the people. The victim is Jack, Donald’s best friend. Donald has brought Jack, who was dating Hilary, the woman who owned Mercury, to visit the horse after hours. Donald is the only one who saw Viv standing in the shadows of the barn holding the gun.

Donald is the narrator again in the third part of the novel as he tries to piece together the circumstances that lead to the tragedy. He struggles to decide if he should turn Viv in to the police as the shooter or if he should leave the identity of the shooter a mystery. Meanwhile, Jack, who is also blind, suffers debilitating stress and fear following the incident. Donald believes telling Jack the truth will help Jack to heal. Instead, Jack is angrier with Donald for keeping the identity of the shooter a secret even while Donald pretends to be Jack’s friend.