Plot
Simon Snow is an 18 year old orphaned magician returning to Watford School of magic for his eighth and final year. Raised among the "Normals" (humans without magical powers) Simon was discovered as a child by his mentor The Mage, who is the headmaster of Watford, and the leader of the magical world. Simon has an unprecedented amount of magical power, and is thus dubbed "The Chosen One", the prophesied Greatest Mage. The prophecy states that one will come to end magic (believed to be a recently appeared creature called the Insidious Humdrum) and one (believed to be Simon) will bring its fall. Around the time of Simon's birth, magical "dead spots" started appearing all over England, places where magic no longer exists, and magical beings are unable to use their powers. Their appearance is attributed to the Humdrum.
The book is also frequently interrupted by short chapters narrated by a character named Lucy, who reflects on her own times at Watford years ago, and her budding relationship with another student named Davy, a man obsessed with the prophecy of the Greatest Mage.
Simon reunites with Penelope and the shy goatherd Ebb, as well as his beautiful girlfriend Agatha Wellbelove. Despite their seemingly perfect relationship, Agatha yearns for independence and breaks up with Simon. Yet another point of contention in Simon's life is his vampire roommate Basilton "Baz" Grimm-Pitch, whom he considers his "nemesis." For the first two months of the school year, Baz is absent with no explanation, to Simon's frustration.
During October, when the veil between the living and the dead is thin, Simon is visited by Natasha Grimm-Pitch, Baz's mother and the former headmistress of Watford before The Mage. With Baz missing, she instead asks Simon to avenge her murder (seemingly committed by vampires) and to find "Nicodemus." When Baz returns, having been kidnapped by creatures called Numpties and only recently escaped, he, Simon, and Penelope try to research Natasha's death and discover who Nicodemus is, but to little success.
Over Christmas break, Simon learns from Ebb that she had a twin brother named Nicodemus; however, after graduating, Nicodemus asked a vampire to turn him in order to gain immortality. His request was fulfilled, but he was banished from the magical world. Simon rushes to Baz's house to inform him, interrupting the Pitch family celebrations.
They track Nicodemus to a bar in the West End, yet he only tells them that a few weeks before the attack, a magician came to him and brokered a deal to have him and other vampires attack Watford. Nicodemus at the time said no, yet refuses to tell Simon and Baz the wizard's name. Baz storms off and, torn at the idea that he has failed, intends to kill himself through self immolation. Simon talks him out of it and kisses him.
Agatha and Penelope travel to Baz's house the next morning at Baz's request. Baz and Penelope explain their investigation to Agatha, but she is wary of going behind the Mage's back and shocked that Simon and Baz are now apparent friends; she leaves with Penelope, while Simon comes back to stay with Baz. They have Christmas Eve dinner with the Pitch family.
After dropping off Penelope, Agatha has a conversation with her mum. Mrs. Wellbelove gossips with her daughter about a witch named Lucy Salisbury who reportedly ran off to America with a Normal after having an illegitimate child, and swore off magic. Agatha becomes entranced by the idea of also escaping her pressuring life to somewhere else.
Meanwhile, Lucy remembers beginning a relationship with Davy and they move in together following their graduation. Davy convinces himself that he is destined to bring about the Greatest Mage, and impregnates Lucy one night, convinced he is the Greatest Mage's father.
The Humdrum attacks Simon at Baz's house and drains the area of its magic, creating a dead spot. However, he claims that he did not take the magic and is merely "what’s left when you're done." Simon uses his powers to sprout wings and flies away to Penelope's house.
The next morning, Baz shows up at the Bunce family's doorstep. While looking at the map of dead spots, Baz shows that they all appeared on the dates that Simon has used powerful magical attacks over the years. He theorizes that Simon cannot simply make more power; he takes it from different locations in Britain without realizing it, thus creating the dead spots. Baz and Penelope also believe that the Humdrum is merely an echo or a hole that Simon created due to tearing out so much magic at once, explaining why the Humdrum looks like an eleven year old Simon. Simon wants to tell the Mage, but Baz is against this and thus Simon travels to Watford alone.
Ebb is arrested by the Mage, causing Nicodemus to join Baz's side, and informs them the identity of the wizard who approached him to as a demonstration of trust: it was the Mage, who hired the vampires to kill Baz's mother in order to take power and incite panic. Baz and Penelope head off to confront him and save Simon.
At Watford, the Mage attempts to take Ebb's magic, wanting to make himself powerful enough to defeat the Humdrum on his own. In the struggle, he stabs her just as Simon arrives. Simon attempts to revive her while the Mage tries to convince Simon to give him his magic, as he believes Simon is not the proper vessel for the Greatest Mage. The Insidious Humdrum appears, and Simon, now understanding of the Humdrum's situation, gives the Humdrum all of his magic in order to "fill the hole." Baz and Penny show up and Simon accidentally kills the Mage. Baz comforts Simon as he sobs over the Mage's corpse.
Lucy planned to name her baby Simon Snow Salisbury. When she went into labour, Davy, now revealed to have been the Mage, performed a ritual to make the unborn child a vessel for unexplainable power. She died in childbirth and Davy hid a newborn Simon among the Normals with the name "Simon Snow" written on the baby's arm. It is shown that the chapters are being told from the perspective of Lucy's ghost, who attempts to visit Simon during the veil lift in October, but is unsuccessful.
In an epilogue, Agatha has moved to California and adopted a cocker spaniel which she has named Lucy. There has been a trial, and Mitali Bunce (Penelope’s mother) is now the new headmistress of Watford. Baz returned to Watford for his final year, but Simon no longer has any magic left, and Penelope dropped out to support him. Baz and Simon are now officially dating and Simon moves into a flat with Penelope.
