The Book of Evidence is the account of Frederick Montgomery, an Irish scientist who has been imprisoned for stealing a painting and the murder of a young housemaid. It outlines his current experiences in jail and looks back on the circumstances that led to his crime.

Montgomery's tale is narcissistic and may hold little resemblance to the truth of his life. He starts as if he is addressing judge and jury at his trial, hoping they will understand his crime, but not asking for forgiveness. He often asks the judge or the jury to consider how they would have felt in his circumstances. There are undertones of psychological abnormality, sexual confusion and callous disregard for the girl Montgomery killed.

Montgomery details an experience on an unnamed Mediterranean Island that leaves him in debt to a violent crime figure and requires him to return to Ireland to find money to repay the debt. His return to his family home Coolgrange finds his mother and a young woman Joanne running a pony school. He argues with his mother because she has sold their paintings. Before he leaves in a rage, his mother tells him that Joanne is like the son she never had.

Montgomery then visits Whitewater, the family home of the Behrenses, who bought the paintings from his mother, and he spies a small Dutch master. He returns the next day to steal the painting. A young maid startles him, and he gives her the painting to carry as he marches her out to his car. She is screaming at him to let her go as he drives away. He stops the car and bashes her with a hammer. He continues driving and later realizes that she is not dead. He stops the car, throws the painting into a ditch and leaves the girl to die.

Montgomery hides out at the home of Charlie French, an art dealer and old family friend. He talks about his childhood, his relationship with Charlie French, his parents, his wife and his child, who has just been diagnosed with some rare mental disease that he can't remember.

Montgomery's solicitor advises him to plead not guilty to murder and at some later time to plea-bargain a lesser charge of manslaughter. Montgomery never maintains that he is innocent of the crime. He always intended to kill the girl. He just can't work out when he decided. He changes his plea to guilty of first-degree murder, and he knows that he will be given life in jail.

Montgomery's interviews with police officers lead the reader to understand that they don't believe his stories. They question his relationship with Charlie French and Charlie's sexuality. The story ends with the detective inspector asking Montgomery how much of it was true. He replies, "All of it, none of it, only the shame." The reader is left with a work of fiction within a work of fiction.