Sixteen-year-old Thomas Fawkes suffers from the Stone Plague, a mysterious illness that slowly kills its victims by turning them to stone. Healers can’t help, and his only hope is passing his Color Test and using color power to command the grey stone to leave his body.
To pass the test, he must receive a mask from his father, Guy Fawkes, and bond with a color. Once he’s bonded, his mask will become the color he’s bonded with. When he wears the mask, he’ll be able to command items of that color.
At the time of the test, Thomas looks for his father and doesn’t find him. Without a mask, Thomas cannot continue his schooling and is cast out. He vows to find his father in London and discover why he refused to come to the Color Test with a mask for Thomas.
During his journey, Thomas is captured and imprisoned for traveling without papers. The next day, the sheriff tells Thomas that he is to be sent back to where he came from. Thomas lies and says he’s from London. He is transporter with other prisoners by cart to London.
The prisoners are Keepers, people who believe that a person should bond with one color only. They are at war with the Igniters, people who believe in communing with White Light. Each group believes the other is responsible for bringing the curse of the Stone Plague down on England.
King James I encourages the capture and death of all Keepers in the hopes that their demise will end the plague. The Keepers tell Thomas that White Light is the only color that speaks back to those with color power. They believe Igniters are power hungry because speaking with White Light allows them to control all colors.
Closer to London, Thomas tries to speak to Grey so he and the Keepers can escape. When it doesn’t work, he tries speaking to White. White speaks back to him and unlocks the prison cart. Thomas escapes into the city.
That night, he gives in to desperation and tries to steal a coin purse from a passing man. The man questions Thomas to find out why he’s in London. Thomas says he’s a friend of Guy Fawkes and was looking for him. The man doubts Thomas and reveals he is Guy Fawkes.
Before Guy can kill him, Thomas reveals his true identity. The two didn’t recognize each other because they hadn’t seen one another since Thomas was very young. Guy tells Thomas that he and his friends plan to kill King James. They believe if they put a Keeper on the throne, the plague will end. Thomas agrees to join the plot in the hopes that it will heal him of the plague.
While waiting for his father to introduce him to the other plotters, Thomas helps save a woman from being attacked by two men. Thomas recognizes her by her mask. Emma was a fellow student from school, and he used to have a crush on her. He escorts Emma home. She sees his plague and is not repulsed by him.
Guy introduces Thomas to Robert Catesby, the leader of the plot. Catesby speaks with Thomas and convinces him to ignore White Light. Thomas agrees and is inducted into the plot. Catesby’s plan is to use gunpowder to blow up Parliament, killing the king and his government.
Thomas meets Emma in the market. Her guardian, the Baron Monteagle, wishes to meet him and thank him for helping her. Henry, his son, is in love with Emma. When Thomas meets the Baron, he asks for a job. The Baron agrees, and Thomas becomes Emma’s new escort.
The Baron is a member of Parliament, and Thomas uses the position to gather information for the plot. Emma and Thomas grow closer as they spend more time together. He learns that she’s seeking an apprenticeship so she can leave the Baron’s home and make a way for herself. She tells Thomas the next date that Parliament is to meet.
Catesby acquires gunpowder for their attack. The group begins digging a tunnel toward the House of Lords where Parliament meets from a nearby house they rented. As the tunnel progresses, they move the gunpowder into the tunnel. After a night of digging, the tunnel collapses and buries the gunpowder. The plot is saved when Guy finds an undercroft beneath Parliament they can rent. All they need to do is unearth the buried gunpowder and move it to the undercroft.
Thomas is instructed to attend the king’s masquerade ball as Percy’s servant. Percy is one of the king’s guards and also part of the plot. Thomas sees Emma at the ball and the two speak quietly in the garden. Emma has never taken her mask off in front of him, but feels comfortable enough to reveal her face to Thomas.
She is African and hides her heritage behind her mask. Henry finds them and is angry. He doesn’t like how she trusts Thomas. So he reveals Thomas’ plague to those at the crowded ball. John Dee, a renowned alchemist, claims he can heal Thomas. He grabs hold of Thomas and heals him of the plague in front of everyone.
The plotters are surprised to see Thomas is healed. They intend to go through with the plan even though killing the king clearly isn’t the way to end the plague. They want Keepers to be free to live in peace.
Catesby inducts Dee into the plot, but Thomas isn’t sure Dee can be trusted. Dee uses his advanced color power to uncover the gunpowder from the collapsed tunnel. While his father is away, Emma and Thomas go out at night and try to free Keepers who have been captured and are being transported for execution.
Though she’s an Igniter, Emma doesn’t believe in killing the Keepers. During one of their missions, Emma shows extreme power when she commands White Light to help them. The display ignites Thomas’ curiosity about White Light.
One night while Thomas is traveling home, he sees a hooded man cutting people with a stone knife. Thomas follows the man and ends up in a struggle with him. Thomas recognizes the man as Dee. Dee cuts Thomas in the shoulder before he escapes. Thomas reaches home and finds the plague is putting roots in the cut.
He realizes Dee and his stone daggers control the plague. The sickness spreads quickly, and Thomas is overcome. Before he dies, White Light speaks to Thomas and says it can help. Thomas invites White in before he loses consciousness.
Thomas wakes weeks later to find that the plague has left him blind, but White did save his life. Dee visits Thomas and threatens to kill his father and Emma if Thomas reveals what he knows. White speaks to Thomas again and shows him how, if he calls out the name of a color, Thomas can see all items around him of that color.
Thomas finds Emma and tells her everything about the plot and how he has become an Igniter. She believes White saved Thomas so he can stop the plot. He believes Emma is right. He writes a letter to the Baron and warns him not to attend the next Parliament meeting. News of the letter reaches Catesby and the plotters though they don’t know who sent it. Rather than discouraging them as Thomas hoped, the letter strengthens their resolve.
The day of the plot arrives, and Thomas and Guy are in the undercroft checking that the gunpowder is ready to be lit. Thomas can’t lie anymore and reveals that he has chosen to follow White Light. Guy uses his color power to trap Thomas among the barrels of gunpowder and leaves.
Thomas tries commanding all the colors, hoping he can get free. When that doesn’t work, he speaks the name White. White purges the Stone Plague from Thomas’ body and uses it to form a white mask for him. Thomas can see again, and he and White are bonded.
When Guy returns, he finds Thomas is free. The two men argue. In the heat of their fight, Thomas reveals that Dee controls the plague. Before they can continue, they are interrupted by a knock. Guy instructs Thomas to hide, and if things go badly, warn the others about Dee. The Baron, Henry, Dee, and soldiers break into the undercroft and find Guy. Dee has betrayed the plotters and vilifies Guy. Thomas watches as his father is taken away.
Thomas enlists Emma’s help in stopping Dee. Together, they warn the plotters still in London, before traveling to the country where Catesby and the others are waiting. Catesby and the others refuse to flee and stand their ground against Dee. The men who aren’t killed by Dee’s plague are taken to the tower to be tortured and killed. Dee almost manages to flee during the battle, but Emma shoots at him, and Thomas uses color power to ensure the bullet kills Dee. With Dee dead, the plague disappears.
For his part in stopping the plot, Thomas is granted a reward. He asks for his father’s life to be spared. The king refuses, so he asks to visit his father and to allow Emma to paint the queen’s portrait. Thomas hopes the queen will give Emma a letter of recommendation so she can live as a painter and leave the Baron’s home. The king agrees. Thomas visits Guy in the tower and sees that his mask has been broken and reattached with tar and nails. Thomas talks to Guy about White Light and begs him to try and speak to White before he is killed. Guy eventually agrees to try.
Thomas and Emma are present at the execution. When Guy is led to the gallows, he stops and rips the broken mask pieces from his face. He never removed his mask before. This is the first time Thomas sees his father’s face. Thomas notes the blood on his father’s face is tinged with white, a sign that Guy has chosen to be an Igniter.
The impatient hangman tries to force Guy up to the gallows, but accidently knocks him from the platform. The fall kill’s Guy. Thomas and Emma leave before the traitors are disemboweled. Emma received the queen’s recommendation, and the two leave London in search of a better life with White Light by their side.