Death Comes to Pemberley picks up six years after Elizabeth and Mr. Darcy from Pride and Prejudice have wed. They have two young boys and are happily married. Elizabeth is welcome in the library at Pemberley and enjoys her life. As is custom, they host an autumn ball at Pemberley annually, which originated with Mr. Darcy’s mother, Lade Anne.

As the house buzzes with preparations for the ball, Lydia Wickham (Elizabeth’s sister who is not invited to the ball because of her husband) arrives at Pemberley unannounced. The chaise she arrives in was seen driving erratically on her approach. She is in a state. She screams that her husband, and Darcy’s long-standing rival, Wickham has been murdered.

Darcy, Fitzwilliam, and others, including Henry Alveston, a young lawyer who is secretly wooing Darcy’s sister Georgianna, set out to find Wickham’s body. It’s a foreboding night and the wind howls. When they arrive in the woodland, what they discover isn’t what is expected. Wickham is alive and kneeling over the corpse of Captain Denny. He cries that he is responsible for Denny’s death.

An investigation into the murder is launched. After alibis are collected from all at Pemberley and the cabin in the woodland, Wickham remains the only suspect. Darcy has conflicting emotions about Wickham’s guilt. At once he is just in his long-standing dislike of Wickham, yet he cannot convince himself that Wickham is capable of murder.

An inquest is held and it is determined that Wickham should stand trial for Captain Denny’s murder. In an effort to get a fair trial, the trial is moved to London. Darcy, among others, is called to testify. He recounts what he saw the night Captain Denny was killed, and reiterates that he doesn’t think Wickham is capable of murder. Much is made of the long-standing feud with Wickham and how Wickham and his wife, Elizabeth’s sister, were not invited to the autumn ball at Pemberley.

Ultimately, Wickham is found guilty of murder and sentenced to the gallows. After the verdict is read, a distraught Eleanor Younge flees the courtroom and flings herself in front of an oncoming horse carriage to kill herself, although her motives are unclear.

Before Wickham is sent to the gallows, a letter arrives at the court and the deathbed confession of William Bidwell is read. Though Bidwell has been bedridden and dying for some time, he encountered Captain Denny, mistaking him for Wickham. It is revealed that Wickham had an affair with Bidwell’s sister Louisa and Louisa ended up pregnant. Louisa was sent to stay with her sister and her husband until the baby was born. Thinking that the child would be raised by Louisa’s sister as her own, Wickham thought the matter of the child was resolved. When Louisa’s sister refused to raise the baby, Wickham arranged to have the child adopted by Eleanor Younge. When Wickham was sentenced to death, Eleanor assumed that the child would never be hers. Devastated at the impending loss, Eleanor committed suicide before she could live to see Wickham pardoned. The child is adopted by another family.

Following Wickham’s pardon, arrangements are made to for Wickham and Lydia to start a new life in America. Darcy helps to pay their passage to America, knowing that if Wickham and Lydia were to stay in England, Darcy would remain financially responsible for them both.

With Wickham and Lydia gone, and Alveston’s true feelings known to Georgianna and the others, life returns to form as spring arrives in Pemberley. During an afternoon stroll, Elizabeth informs Darcy that she is expecting another child.