ll Stanton, the youngest of 10 children, will celebrate his 11th birthday on Midwinter's Day. On the day before his birthday, an old man hides and watches him, and a neighbor, Farmer Dawson, warns Will about a man called the Walker and impending danger. He does not offer an explanation but gives Will an iron circle divided equally into four sections by a cross and tells him to keep it with him at all times.
On his birthday, Will wakes up to find that his surroundings are different. Trees have replaced buildings, and roads are now only tracks. Will meets a Rider, but refuses the Rider's food and to sit on his black horse. Will narrowly escapes when the Rider tries to take him. A white horse allows Will to carry him, when Will asks, and Will instinctively knows that he must find someone called the Walker.
The old man from the day before is the Walker. When Will tries to get answers from the Walker, the Rider attacks. The white horse saves him, and the Walker runs away. The white horse takes Will to a hill with a freestanding door that seemingly leads to nowhere.
Will goes through it and enters a great hall. He meets Merriman Lyon and the Lady. They explain that like them, he is an Old One, an immortal being that protects the world from being taken over by the forces of evil called the Dark. Will is the last of the Old Ones and has been tasked with finding six magical signs that will help the Old Ones in the battle between Dark and Light. Farmer Dawson gave Will the first sign, the sign of iron, and he now has to find the other five: the signs of bronze, wood, stone, fire and water.
Merriman explains that they are not in Will's time but hundreds of years in the past, which is why the surroundings look so different. Merriman returns Will to his time and tells him that he will get the second sign from the Walker.
A few days later, on Christmas Eve, Will is returning from shopping when he sees a dead branch in the road and decides to set it on fire, a trick Merriman taught him in the great hall. As the branch is blazing, the Walker accosts Will from behind and demands that he put out the fire because it will attract the Dark. Will asks for the second sign from the Walker, who tells him that he has been forced to carry the sign for a very long time but fears giving it to the wrong person. Will convinces the Walker to give him the sign and as he does, Maggie Barnes, the dairymaid from Dawson's farm, approaches them.
Too late, Will realizes that Maggie is an agent of the Dark. She uses magic to paralyze him and takes both signs, but Merriman appears and uses the magic of the Light to stop Maggie and save Will. Merriman admonishes Will for playing with his power and attracting the Dark. Will promises never to abuse his power again.
On the night before Christmas, the Stanton children go caroling at a large manor owned by Mrs. Greythorne. Will is surprised to see Merriman working as the manor's butler but soon discovers that Mrs. Greythorne is also an Old One. While the children are singing to Mrs. Greythorne, Merriman stops time and takes Will back a few hundred years to a party in the mansion where they observe a ritual to remake the sign of wood. After the ritual, the new sign is hidden in paneling on the wall.
Merriman introduces Will to Hawkin, a man whom he has raised as a son and considers a confidant. When Merriman betrays Hawkin's trust, Hawkin becomes an agent of the Dark. Merriman returns Will to his own time, and Will retrieves the sign of wood from the paneling where it was hidden.
On Christmas morning, Will feels an impending attack of the Dark waiting for him outside the church. He is helped by Old Ones who live close to the church, and together they push back the force of the Dark. He finds the fourth sign, the sign of stone, in the church. Wild birds lead Will to find the Walker collapsed in the churchyard.
Will and Paul take the Walker back to their home where Mrs. Stanton nurses him back to health. As the weather gets increasingly worse, Merriman suggests that all the townspeople gather at Mrs. Greythorne's manor to wait out the storm.
While at the manor, Will travels back in time and sees the Lady who warns him that the Dark is holding the entire country in a grip of cold and snow, and Will must get the sign of fire to break that grip.
Will learns that the Walker is actually Hawkin, who, because of his betrayal to the Light, has been cursed to carry the second sign for hundreds of years until the last Old One, Will, takes it from him. Will uses flames to create the sign of fire, causing rain to fall and the snow and cold to stop.
