Midnight Magic by Avi is a fantasy novel set in the imaginary kingdom of Pergamontio in 1491. Mangus the Magician has recently been placed on trial for using magic and has sworn in return for his life that he will never do magic again. However, when a ghost begins appearing to the king's daughter, Mangus is brought to the castle in hopes that he will be able to solve the mystery of the princess's nightly hauntings before news of these visitations become public knowledge. Mangus finds himself caught between the fact and reason as he attempts to learn the truth of the princess's stories. Midnight Magic is a mystery set in a fairy tale world that will leave readers glued to the page until the final sentence.

Fabrizio is studying tarot cards when a castle guard knocks on the door with a letter from Count Scarazoni, the same aristocrat who had Fabrizio's master, Mangus the Magician, tried for using magic not long ago. Fabrizio fears for his master's life when he learns that Mangus has been summoned to the king. Fabrizio begs Mangus to allow him to go to the castle with him.

When Mangus and Fabrizio arrive at the castle, they learn that the princess has been seeing a ghost in the corridor outside her bedchambers for nearly month. Mangus is charged with finding out if the ghost is real or an illusion. Mangus announces to the court that he does not believe in ghosts, but that he will investigate the princess's story. Mangus and Fabrizio are then taken to a bedchamber for the night. Mangus sends Fabrizio out to get water. Fabrizio quickly gets lost and runs into the princess. It is then that Fabrizio sees the ghost for the first time. However, the princess makes him promise not to tell Mangus what he has seen.

The following day, Fabrizio is sent to the kitchen to get food for his master. While there, Fabrizio meets Rinaldo, a kitchen servant. Rinaldo takes Fabrizio into the king's chapel to hide from the count. While they are there, the count comes into the chapel. Fabrizio hides behind the altar when Rinaldo disappears behind a secret door. Fabrizio hears the count discussing with the princess's tutor a plot to kill the prince and marry the princess.

Fabrizio returns to his bedchamber and tells Mangus what he has heard. Mangus and Fabrizio are then escorted to the princess's rooms to interrogate her. It is during this questioning that Mangus learns that the princess believes her brother, who is supposedly in Rome, has been murdered. A short time later, Mangus goes to investigate the corridor where the ghost has been seen. Mangus finds a piece of wood that he believes came from a statue that once stood in that niche. Mangus sends Fabrizio to find the statue, one that he quickly finds in the quarters of Rinaldo, the kitchen servant.

Fabrizio tricks Mangus into going to view the apparition that night at midnight. When the ghost appears, it speaks to Mangus, asking that Fabrizio avenge his murder. Later that night, Fabrizio meets with the princess. Afterward, Fabrizio is arrested for trying to steal state secrets from the princess. Fabrizio learns the next morning that the princess has made these accusations against Fabrizio so that he might have the freedom to move around the castle through secret passageways while the count believes he is locked in the dungeon.

Mangus, Fabrizio learns, has figured out how the apparition keeps appearing. Mangus takes Fabrizio and the princess to the family chapel where he shows them a candle, mirror, and silver plate that creates the green light that appears to be a ghost. The queen joins them and Fabrizio and Mangus learn that the count sent the prince to Rome and then had someone kill him. With no evidence to convince the king, the queen and princess have come up with the ghost to attempt to stop an upcoming wedding between the princess and the count. Together with Mangus and Fabrizio, the queen and princess agree to a plot to convince the king of the count's guilt.

The following day, Mangus sets up a magic show that is intended to showcase the ghost with Fabrizio creating the illusion. Fabrizio is to accuse the count of murder to convince the king of his guilt. However, when Fabrizio arrives at the chapel to create the illusion, he finds Rinaldo there. Rinaldo is really the prince in disguise and he has destroyed the candles in order to appear himself as the ghost to force a confession. However, the prince has injured his leg. Fabrizio stands in his place, forcing a confession that leads to the counts arrest.