At Gloucester's house, a Witch and Bolingbroke, a conjurer, arrive with Hume. They discuss the Duchess and suggest that she watch their work from above, with Hume. The Duchess enters above and greets them. They begin their ceremony with Bolingbroke's incantations. A spirit appears and says that he will answer their questions. Bolingbroke reads from a list of questions, asking first what will become of Henry. The spirit responds: "The Duke yet lives that Henry shall depose, / But him outlive, and die a violent death" (I.iv.29-30). Bolingbroke asks about the fate of Suffolk, who the spirit says will die at sea. The spirit says Somerset should avoid castles. Then, the spirit sinks into the ground, with thunder crashing.
York and Buckingham enter with soldiers. York orders the arrest of the conjurers. Finding the written questions, Buckingham orders the Duchess' arrest. All are led away, and Buckingham and York read Bolingbroke's questions. Buckingham asks if he may ride to Saint Albans and tell the king and Gloucester about the Duchess' arrest.
Even if Gloucester is an honorable man, he is alone among a crowd of plotters and schemers. Even his wife has designs upon the throne. It is her ambition that Beaufort and Suffolk will use to begin bringing Gloucester down, cleverly paying off Hume to urge her to make use of occult forces.
The petitioners who seek Gloucester firmly believe in the power of the upper classes to put things right in the lives of the common people, and they want Gloucester to help fix their problem. Yet mistaking Suffolk for Gloucester provides Suffolk with an opportunity to weaken York, while it ruins the petitioners' chance to solve their grievances. Margaret speaks of the surprising weakness of Henry; it is clear to her that she must gain power in other ways than through her husband. Margaret and Suffolk want to bring down everyone in each faction; they have a long struggle for power before them.
Gloucester advises justice will be served in the case of Peter and Horner if the two men engage in single combat, which startles Peter. After all, he only wanted to get his master in trouble, he didn't want to fight. This kind of trial shows the idea of justice has become a farce, and the lords think it's easier to have the petitioners fight it out than to listen to their complaints and hold a real trial.
The debate between Somerset and York to be regent of France refers to a time during the French wars, when York was regent of France and Somerset delayed sending troops to York. York says if he were to become regent now, France would surely fall because of Somerset's lack of aid.