The following version of this book was used to create this study guide: Phillips, Arthur. The King at the Edge of the World. New York: Penguin Random House, 2020.

In Part I of The King at the Edge of the World, the author introduces Mahmoud Ezzedine. It is 1591 and Ezzedine is the doctor to the sultan of the Ottoman Empire. He lives in Constantinople with his wife and their young son. The sultan asks him to accompany the ambassador and his adviser, Cafer bin Ibrahim, on an upcoming journey to England to meet Queen Elizabeth I. Once there, Ezzedine finds England very strange and filthy and longs to return home. While at the palace, Ezzedine speaks to a poet espousing atheism. When Cafer bin Ibrahim learns of this conversation, he threatens to tell the sultan. He wishes for Ezzedine to be arrested or even executed because he is interested in Ezzedine's wife. At a palace party shortly before the diplomatic entourage is supposed to return to Constantinople, Ezzedine intervenes when one of the English noblemen, Baron Moresby, has a seizure. Queen Elizabeth sends Moresby back to his country home in northern Cumberland and asks the ambassador if Ezzedine might accompany him. Believing this will help Ezzedine avoid potential trouble with the sultan, the ambassador agrees.

Part II introduces Geoffrey Belloc, an English spy. It is 1601. Belloc visits Robert Cecil, the queen's secretary, and tells her that he and several other spies and prominent members of government are concerned that King James IV of Scotland is a Catholic. This is concerning because James is next in line to succeed Elizabeth and she is ill. There has been a decades-long history of violent conflict between Catholics and Protestants in western Europe at this time, which is described briefly in flashbacks to Belloc's earlier life. He saw a mob of Catholics burn a Protestant alive when he was just four years old, and as a spy he infiltrated covert groups of Catholics and learned about their plots to overthrow the government. One of these plots involved King James's mother, Mary, Queen of Scots, who with her followers planned to assassinate Elizabeth and put Mary on the English throne. Belloc was integral in Mary's arrest and execution. Robert Cecil gives Belloc permission to organize a plot to determine whether or not King James is Catholic like his mother. As Belloc tries to imagine who he might send to infiltrate the king's palace, he recalls Mahmoud Ezzedine attending to Moresby during his seizure.

In Part III, Ezzedine is still living in Cumberland with Moresby. He is now called Matthew Thatcher, as he converted to Christianity and changed his name before leaving London, hoping to fit in better with the English. Ezzedine has long ago stopped dreaming of returning to Constantinople, because thinking about home and his family is too painful. Belloc arrives in Cumberland and introduces himself to Ezzedine as David Leveret. He tells Ezzedine about the situation with King James and the succession and promises Ezzedine he will send him home to Constantinople if he succeeds in determining whether or not James is Catholic. Ezzedine agrees.

In Part IV, it is 1602 and Ezzedine arrives in Edinburgh under the auspices of being a gift from Queen Elizabeth to her cousin, King James. He is not introduced to the king for weeks, but when they finally meet, Ezzedine piques James's interest when he mentions that he often played chess with the sultan back in Constantinople. The two men begin playing chess together regularly, and Ezzedine attempts to determine James's religious beliefs during their conversations. However, this proves impossible. Belloc and Ezzedine then decide to initiate a second version of their plan in which Ezzedine will poison the king (with the deadly substance applied to the chess pieces, as James has a nervous habit of touching the pieces and then licking his fingers) to see if he will make a deathbed confession to a Catholic priest. If he does not do so, he will be deemed Protestant and Ezzedine will administer an antidote to the poison. All goes relatively according to plan, but while the king is ill, Ezzedine steps out to retrieve the antidote and James calls in the priest during his absence, making the confession unseen. Ezzedine assures Belloc that James made no confession and is therefore Protestant. Belloc tells Ezzedine he will return him to Constantinople as promised.

In the epilogue, Belloc returns to London and tells Robert Cecil that he is certain King James is not Catholic. The narrator notes that James had been informed of the poisoning plot and went along with it to prove to anyone questioning him that he was not Catholic because he was concerned about appearing legitimate as a potential successor to Queen Elizabeth. Ezzedine returns to Cumberland, but on his way from Cumberland to London, he is murdered in a tavern by King James's men. In an alternate ending, he does not stop at the tavern and therefore makes it safely to London and then Constantinople. In the final chapter of the epilogue, God shows Ezzedine all of the choices he might have made throughout his life and to what fate they would have led him.