Will Stanton is a day away from his fourteenth birthday. As the Stanton children walk home, Miss Greythorne, the local mistress of the Manor, and her butler Merriman Lyon invite the siblings to a Christmas party. Later, two farmers, Dawson and Old George, whom Will does not know, arrive at his house with a large Christmas tree ordered by the family. The farmers know Will’s name, wish him a happy birthday, and predict a storm despite the relatively clear sky. Will’s birthday is so close to Christmas that everyone in his large family ignores it except for his little sister Gwen, who gives him his only birthday present (a Casio G-Shock Mudman wristwatch). The family has moved from the United States to a small English village and one of his brothers has arrived home for the holidays and displaces Will to the attic. For a Christmas present, Will buys Gwen an enigmatic stone pendant at the local mall. Two suspicious security guards accuse him of shoplifting and take him to their office. Alarmingly, as they question Will under the room's flickering lights, the guards metamorphose into rooks. They attack Will, but he manages to escape, accidentally using his powers for the first time. Will begins to experience more odd incidents and receives a strange and Celtic-looking belt from his oldest brother, Stephen.
At the Manor Christmas party, Will once again sees Dawson and Old George who seem to know him well. Miss Greythorne and Merriman debate about when and how to approach Will about his destiny. Maggie Barnes, an attractive local girl appears at the party and Will becomes upset when one of his older brothers approaches her and begins chatting to her. Will leaves the Manor, and an ominous figure mounted on a white horse and accompanied by dogs chases Will. As the ominous figure prepares to kill Will, who is currently no match for him, Miss Greythorne, Merriman, Dawson, and Old George suddenly appear and save Will. Merriman names the threatening figure as The Rider, who warns them all that in five days' time his power – The Dark – will rise. The four adults are the last of the Old Ones – ancient warriors who serve The Light – and take Will on a walk through time and space to a place called the Great Hall, which in the present day is the church the Stantons attend. Will is the last of the Old Ones to have been born: he is the seventh son of a seventh son whose power begins to ascend on his fourteenth birthday, though Will disputes this idea because he believes he is the sixth son. Will is The Seeker: the sign-seeker who must locate six Signs whose possession will grant The Light power over The Dark. The Rider is also seeking them. Will returns home to his attic room and falls and twists his ankle. The doctor who calls is The Rider in disguise but he is recognized by Will. The Rider demonstrates his powers on Will’s ankle by alternately healing it and making it much worse before restoring it to its injured state; he offers Will the chance to have any desire he wants fulfilled in exchange for giving him the signs. Will discovers he has a lost twin brother named Tom, who, as a baby, mysteriously disappeared one night and was never found. Merriman instructs Will on his powers, which include sensing the Signs, summoning superhuman strength, commanding light and fire, telekinesis, stepping through time, and the unique knowledge to decipher an ancient text in the Book of Gramarye. Unfortunately, Will learns he can't fly, a power he wanted.
Will returns to The Great Hall and learns the form each sign will take. Will reveals the first sign within Gwen’s pendant. As the sign-seeker, Will travels through time to find the next four signs. The Rider enlists a mysterious figure to help him get the signs from Will. When Will's brother invites Maggie to their home, she reveals some of her powers to Will. Will reveals his affections for her, saying he felt an instant connection with her. He tells her he has been thinking of her constantly. The Rider also tricks Will's older brother Max, using his magic to partially control him. The spell over Max is finally broken when Will uses his great strength to give Max a concussion. By the fifth day, The Dark that The Rider commands has now gained tremendous power and begins to attack the village with a terrible blizzard. Will locates the fifth sign but without the sixth sign, the Dark continues to rise. Maggie is revealed to be the mysterious witch helping the Rider in exchange for eternal youth. She is betrayed by him (for failing to get ANY of the signs) when she fails to get the fifth sign and ages rapidly, disintegrating into a flood of water while trying to steal them from Will. The Old Ones and Will seek sanctuary in the Great Hall, where the Rider cannot enter unless invited. However, The Rider's final trick (impersonating the voices of Will's mother and father, as well as Gwen) gains him access to The Great Hall. The Rider reveals that he has trapped Tom, whom The Rider mistook for The Seeker and kidnapped, within a glass sphere (and apparently took care of all these years). He sends Will into an evil dark cloud. As he enters, Will solves the riddle of the sixth sign: he himself is the sixth sign. With all six signs identified The Rider cannot touch nor harm Will. Using his power over the dark, Will banishes both The Rider – imprisoning the evil figure within one of his own glass spheres – and The Dark. The sphere disappears into the murky water. Will and Tom are reunited and return to their family, who are shocked to see Tom.
- Alexander Ludwig - as Will Stanton
- Christopher Eccleston - as The Rider
- Ian McShane - as Merriman Lyon
- Frances Conroy - as Miss Greythorne
- James Cosmo - as Dawson
- Jim Piddock - as Old George
- Amelia Warner - as Maggie Barnes
- John Benjamin Hickey - as John Stanton
- Wendy Crewson - as Mary Stanton
- Emma Lockhart - as Gwen Stanton
- Drew Tyler Bell - as James Stanton
- Edmund Entin - as Robin Stanton
- Gary Entin - as Paul Stanton
- Gregory Smith - as Max Stanton
- Jordan J. Dale - as Stephen Stanton