Franz breaks off his engagement to Valentine, and Noirtier writes yet another will, this one restoring Valentine’s inheritance. Meanwhile, Fernand visits Danglars to discuss the marriage between Albert and Eugénie. Fernand is surprised and offended when Danglars says he wants to postpone the marriage between their children. When Fernand objects, Danglars says he must break off the planned marriage entirely but refuses to explain why.

Noirtier sends for Maximilien. When he arrives, Valentine announces that her grandfather plans to leave his son’s house for an apartment of her own. Valentine will go and live with him if she receives her parents’ permission. When she is free of her parents, she will marry Maximilien. The meeting is interrupted by the collapse of Noirtier’s servant, Barrois, after Valentine gives him a glass of lemonade intended for Noirtier. Valentine calls for the doctor, who is visiting Edouard, and helps Maximilien to leave undetected.

The doctor tends to Barrois but is unable to save him. After Barrois’s death, the doctor tests the lemonade and finds that it has been poisoned with the same substance he prescribes for Noirtier in smaller doses. The doctor tells Villefort that he believes there is a poisoner in the house, someone who has killed the Saint-Mérans and has now targeted Noirtier.