Several lords, including Richard Plantagenet, Warwick, Somerset, Suffolk, and Vernon, enter the Temple Garden near the law courts in London. Suffolk says it is better to decide matters in the garden because the lords were too noisy inside the law court where they were just disputing. Somerset asks Warwick to decide between him and Plantagenet, but Warwick says that, while he can arbitrate many situations, he cannot decide between these two lords. Both Plantagenet and Somerset declare that the right choice seems shockingly clear to them.
Plantagenet has presented a case for his noble birth; he now says that those who believe he has pleaded the truth should pluck a white rose off the briar to show their allegiance to him. Somerset says those who support him in his dispute of Plantagenet's claim should pluck a red rose. Warwick says he doesn't like colors and he plucks a white rose with Plantagenet. Suffolk says he believes Somerset and picks a red rose. Vernon urges them to stop plucking until they discover which side has more people on it, and both Somerset and Plantagenet agree to yield if they have fewer supporters. And with that, Vernon picks a white rose. Then, a lawyer picks a white rose, too, saying that he believes Plantagenet's case is more correct in the law.
Plantagenet and Somerset trade insults about their flowers and scorn each other. Somerset criticizes Plantagenet's father, who was put to death as a traitor by Henry V. Plantagenet says his father was accused and put to death, but his treason was never proven. He says he will remember this slight for a long time, and Somerset should expect to see results from his insults in future dealings with Plantagenet. Somerset welcomes such a future and says his allies will wear a red rose to remind him of this disagreement. Plantagenet, too, says he will wear the white rose with his faction as a marker of his continued hatred for Somerset. Somerset departs.
Plantagenet and Warwick talk; Warwick says he believes that the next Parliament will restore Plantagenet to the title he lost when his father was put to death. Meanwhile, he will continue to wear Plantagenet's rose, though he foresees that this small brawl in the garden will send thousands of people to their deaths, all in the name of the white and the red rose. The two nobles exit.
In a cell in the Tower of London, Mortimer awaits his death, speaking of his declining strength and wondering when his nephew Plantagenet will come. Mortimer comments on the misfortune he has suffered since Henry V first came to power, and he says that Plantagenet has suffered under the same fate. Plantagenet arrives at the cell, and Mortimer asks him to relate how he came to be in his recent argument.
Plantagenet says he has had a disagreement with Somerset, who criticized his dead father. Plantagenet asks Mortimer to explain how his father came to be executed. Mortimer says the same deed that caused him to be in the tower all these years was the reason for Plantagenet's father's demise. Mortimer explains that his family was next in line to the throne after Richard II, but because Henry IV deposed Richard, Henry's line came to power instead. When he attempted to reassert himself as the rightful heir, Mortimer was thrown in jail. Later Plantagenet's father raised an army to try to install Mortimer on the throne, but he was captured and executed, and the Mortimers were suppressed.