Innocent Blood is a novel by the author P.D. James. In this novel, Philippa Palfrey has learned that her biological mother is Mary Ducton, a murderess who is about to be released from prison. In an attempt to learn more about her biological mother, and perhaps to help her figure out her own identity, Philippa arranges to share a flat with her mother for a few months. At the same time, the father of the young child Mary Ducton murdered is planning to exact revenge for his child's death. Norman Scase follows Mary Ducton out of prison and learns of her relationship with Philippa. Norman uses this relationship to find where Mary Ducton is living and to kill her. Innocent Blood is a novel about revenge, about family, and about learning one's own identity. Innocent Blood is both tragic and inspiring, a novel that will entertain readers of all genres.

Philippa Palfrey has filed under a new law to get her original birth certificate and learn the identities of her birth parents. Philippa has been told that her mother died when she was born, but she hopes to use the information to find her biological father. When the birth certificate comes, Philippa is shocked to find out that her parents are notorious child killers and that her mother is still alive, set to be released from prison within a few weeks. Philippa confronts her adoptive parents with this news, accusing them of lying to her since her adoption. They try to explain that they only wanted to protect Philippa, but she refuses to listen.

Philippa writes to her biological mother, Mary Ducton, and visits her in prison. Philippa decides she wants to share a flat with her mother for a few months in order to get to know her before she goes up to Cambridge. Philippa's adoptive father, Maurice, is dead set against this, insistent that Philippa will only be hurt. However, Philippa remains defiant and begins searching for a flat with the money she has saved for a trip abroad. Philippa finally finds a place, above a green grocer, that she spends a great deal of time fixing up for her mother.

At the same time, Norman Scase has just retired from his job and places his home on the market in order to free him to devote all his time to a plan of revenge. Norman's daughter was killed ten years ago while on her way home from a scouting meeting. Norman's wife had always wanted to kill their daughter's murderess upon her release from prison, but died of cancer before she could. Norman promised he would fulfill her dream of revenge when the time came. Mary Ducton is to be released from prison in a few weeks.

The day Mary Ducton leaves prison, Philippa is there to meet her and take her home. Norman follows at a short distance, watching the two together as they ride the train to London. Norman manages to see Philippa's name on a tag attached to her bag as they disembark in London, but loses the ladies as they go to board another train. Norman concentrates on finding Philippa, finally locating her through a few phone calls to multiple Palfrey households. Norman then sets up watch in a hotel across the street to watch Philippa's adoptive mother and to hopefully learn where Philippa and Mary Ducton are living.

Philippa and Mary Ducton settle into life together. Mary Ducton refuses to talk about the past and Philippa convinces herself that Mary Ducton's crime is not important. However, Philippa cannot keep from reading a manuscript about the crime that Mary Ducton has written. Philippa feels empathy for Mary Ducton and decides that the past does not matter. Together Philippa and Mary Ducton face the future, working side by side and hoping to build an affectionate relationship. At the same time, Philippa's adoptive mother feels pushed out, neglected, and through these feelings accidentally takes Norman straight to Philippa's front door.

Norman watches Mary Ducton closely and schemes to get into her apartment and kill her. Norman steals keys to get into the house, but cannot find a way to enter the flat itself. A few days later, Philippa herself breaks the lock on the front door in order to keep a reporter from harassing them. Afterward, Philippa visits her adoptive parents' home to get some money for an escape, but instead learns the truth about her adoption. Philippa had assumed Mary Ducton gave her up when she went to prison, but Philippa learns that Mary Ducton gave her up more than a year previous to her crime.

Philippa argues with Mary Ducton and leaves her alone in the flat. When Philippa returns late that night she finds Norman in Mary Ducton's room. Mary Ducton is dead, having taken an overdose of pills. Philippa sends Norman away and calls Maurice, her adoptive father, to deal with the death. Philippa returns to her adoptive parents and resumes the life she had before Mary Ducton. Eventually Philippa goes to college and writes a novel. Norman falls in love and marries, hoping for children and a second chance at a happy life.