In 1857, forty-five-year-old Charles Dickens [Ralph Fiennes] notices eighteen-year-old Ellen "Nelly" Ternan [Felicity Jones] while she is performing at London's Haymarket Theatre. He later casts her, along with her mother and sister Maria, in a performance of Wilkie Collins's The Frozen Deep at Dickens's Free Trade Hall in Manchester. At a party following the performance, Dickens and Nelly share a moment alone.
Nelly and her family attend one of Dickens's readings at the Harrow Speech Room. Delighted to see Nelly again, Dickens takes the Ternans to Doncaster Racecourse. No longer having feelings for his wife Catherine Dickens, who is busy taking care of their ten children and does not share his passion for literature, Dickens falls for Nelly. Nelly, in turn, is fascinated by Dickens's fame and ideas.
One morning, Dickens slips out of Gads Hill Place, his country home, and walks to East London to see Nelly in a play. Her mother invites him to their cottage. Noticing the looks between them, Mrs. Ternan cautions him that she cannot afford to risk Nelly's reputation. Dickens responds that he has no intention of compromising Nelly's name. After organising a reading and fundraiser to benefit London's "fallen women" and their children, Dickens invites the Ternans to his town house.
Mrs. Ternan thinks that this blossoming relationship may offer Nelly the financially stable future she would not find on stage, believing that Nelly is not as talented as her sisters. Overhearing her, Nelly is angered and confused by her mother's plans for her to become the mistress of a married man. Catherine mistakenly receives Dickens's gemstone bracelet for Nelly and is ordered by Dickens to bring it to Nelly. Catherine warns her of the pain of being with Dickens, while Nelly is disturbed by his cruelty.
Dickens and Collins arrive late to Nelly's birthday party and talk her into visiting the house that Collins shares with his mistress, Caroline Graves. After demanding to leave, she confronts Dickens about insulting her honour by bringing her there and his suggested arrangement of her being his whore. After arguing that he no longer loves Catherine, Dickens asks to enter Nelly's home and kisses her.
On a walk, Dickens and Nelly have an encounter with his son, who is saddened to see her affections with his father. Dickens later announces in The Times his "amicable" separation from Catherine while denying the rumours of his affair. The news devastate his wife and children.
Mrs. Ternan assures Nelly that Dickens is an honourable man, while Collins tells Nelly that he is a great man and that she has a choice. Nelly argues that only men have the freedom to choose and suggests that she has few other options in life. He urges her to be with Dickens and "break conventions". Nelly visits Dickens, who excitedly reveals that he completed Great Expectations. Nelly expresses her approval of the ending, which does not bring Estella and Pip together. They become lovers.
Nelly becomes pregnant. She and Dickens speak of the gossip of their affair spreading from city to city. Their child is stillborn. After saving a lock of the child's hair, Dickens signs the death certificate with a false name, "M. Charles Tringham".
After returning to England from France in the spring of 1865, Dickens and Nelly board a train at Folkestone for London. Near Staplehurst in Kent, the train derails. As he speaks to Nelly, the workers ask the uninjured Dickens to assist with wounded passengers in the wreckage. He pretends he was travelling alone, to avoid public scandal. As he walks away to help, she watches him retrieve a page from a work in progress.
Dickens installs Nelly at a country house and promises to visit twice weekly, though acknowledges that his family has a claim on his time. Nelly remains his secret mistress until his death in 1870. In 1876, she marries Oxford graduate George Wharton-Robinson, who is twelve years her junior and unaware that she was once the mistress of Charles Dickens. They have a son together. Only Reverend Benham knows about the affair and Nelly's real age. In 1883, while watching her son perform in a school play, Nelly recalls the lines she spoke in The Frozen Deep.
