The following version of the book was used to create this study guide: Wheeler, Jeff. The Queen's Poisoner (Kingfountain Series). North Seattle, an Amazon trademark, 2016. Kindle AZW file.

Owen Kiskaddon is the eight-year-old son of Lord and Lady Kiskaddon, the rulers of Tatton Hall in Westmarch. He is the youngest son. Though Owen was, at first, thought to be stillborn, he revived while his parents were holding him and saying their farewells. Owen has a white streak in his hair, which may have been a sign of that tenuous birth or may be a symbol that he is Fountain-blessed. Those who are Fountain-blessed have been supernatural, mysical powers. The kingdom is ruled by King Severn, who overthrew his nephews in order to take the throne. Severn is a cruel and demanding ruler. When Lord Kiskaddon failed to take his men into battle on Severn's orders, Severn retaliated by ordering the death of Owen's older brother, Jorganon. He also demanded that that Lady Kiskadon choose another son to become the king's prisoner. The child's imprisonment is meant to ensure that Lord Kiskaddon's is loyal in the future.

Owen is terrified at this turn of events, especially when the butler makes it clear that Owen will likely be killed in the near future. A kind cook, Liona, gives Owen information that allows him to escape to a nearby sanctuary, Our Lady of Kingfountain, where the deposed queen lives. Owen reaches Our Lady without being caught, but King Severn arrives just moments after Owen makes contact with the queen. Though the queen warns Owen not to let Severn touch him, Owen does not realize the danger and he falls under Severn's power. Severn is Fountain-blessed and, with a touch, can convince anyone to do whatever he wants. Within seconds, Owen feels at peace with the decision to return to the castle with Severn.

Owen soon meets Ankarette, the midwife who had attended his birth. She is hiding in the castle without Severn's knowledge or permission by keeping to hidden passageways and a secret tower. Ankarette has served as the queen's poisoner for years, and she accidentally poisoned Severn's wife and unborn child. She helps Owen cope with his fears and loneliness while developing a plan to keep Owen safe in the future. She begins an alliance with a butler and a member of the Espion (a network of spies) to gather information that will make Severn think that Owen is Fountain-blessed with the gift of sight. A spy named Mancini provides information that Ankarette crafts into a dream. Owen then tells Severn about this “dream,” though it is just a ruse. When this dream and a vision seem to come true, Owen has Severn's full attention. His future is not yet secure, but it has become less tenuous.

Meanwhile, a young girl named Elysabeth Victoria Mortimer (later nicknamed Evie) arrives at the castle with the news that she and Owen will someday marry. She is pushy and loud, which Owen initially finds to be annoying. Later, he becomes dependent on her fierce loyalty and determination, which gives Owen the courage to do tasks that frighten him. When Owen and Evie are separated, Evie cuts off her braid to give Owen, and that becomes his talisman whenever he is afraid.

Owen travels with Severn and a group of soldiers and advisers to Beestone. There, Owen has the chance to read a book that gives him pieces of history he was missing up to this point. Owen also uses his imaginary gift to save his parents from death, though Severn does banish them from their home. It is also while he is at Beestone that Owen discovers he actually is Fountain-blessed and that Ankarette was only trying to buy him time for the power to manifest. As the novel comes to an end, Owen is named the new Duke of Westmarch in his father's place and is reunited with Evie.