King Edward III is informed by The Count of Artois that he, Edward, was the true heir to the previous king of France. A French ambassador arrives to insist that Edward do homage to the new French king for his lands in Guyenne. Edward defies him, insisting he will invade to enforce his rights. A messenger arrives to say that the Scots are besieging a castle in the north of England. Edward decides to deal with this problem first. The castle is being held by the beautiful Countess of Salisbury, the wife of the Earl of Salisbury. As Edward's army arrives, the rampaging Scots flee. Edward immediately falls for the Countess, and proceeds to woo her for himself. She rebuffs him, but he persists. In an attempted bluff, the Countess vows to take the life of her husband if Edward will take the life of his wife. However, when she sees that Edward finds the plan morally acceptable, she ultimately threatens to take her own life if he does not stop his pursuit. Finally, Edward expresses great shame, admits his fault and acquiesces. He dedicates himself to use his energies to pursue his rights and duties as king.
In the second part of the play, in several scenes reminiscent of Henry V, Edward joins his army in France, fighting a war to claim the French throne. He and the French king exchange arguments for their claims before the Battle of Crecy. King Edward's son, Edward, the Black Prince, is knighted and sent into battle. The king refuses to send help to his son when it appears that the young man's life is in danger. Prince Edward proves himself in battle after defeating the king of Bohemia. The English win the battle and the French flee to Poitiers. Edward sends the prince to pursue them, while he besieges Calais.
In Poitiers the prince finds himself outnumbered and apparently surrounded. The play switches between the French and English camps, where the apparent hopelessness of the English campaign is contrasted with the arrogance of the French. Prince Edward broods on the morality of war before achieving victory in the Battle of Poitiers against seemingly insurmountable odds. He captures the French king.
In Calais the citizens realise they will have to surrender to King Edward. Edward demands that six of the leading citizens be sent out to face punishment. Edward's wife, Queen Philippa, arrives and persuades him to pardon them. Sir John Copland brings Edward the king of the Scots, captured in battle, and a messenger informs Edward that the English have secured Brittany. However, the successes are undercut when news arrives that Prince Edward was facing certain defeat at Poitiers. King Edward declares he will take revenge. Prince Edward arrives with news of his victory, bringing with him the captured French king. The English enter Calais in triumph.

Question: Which king lost to King Edward?
Answer: The King of Bohemia.
Alternative answer: the king of the Scots