A man desperately tries to flee a knight through a dense forest but is ultimately caught and struck with a sword before he inexplicably vanishes. He is later found barely clinging to life in the barren desert, where he utters only a single word: Castlegard. Unfortunately, he succumbs shortly after being rushed to a hospital, leaving behind a pendant marked with the insignia of ITC Corp. Doctors find that his X-ray images depict a disturbing misalignment of internal organs, veins, and bones. Soon enough, an employee from ITC Corp. appears, identifying the deceased as Vincent Taub, and swiftly covers up the incident.
Meanwhile, Professor Edward Johnston (Billy Connolly) leads a team of archaeologists, including his son Chris (Paul Walker), Kate Erickson (Frances O'Connor), Josh Stern (Ethan Embry), François Dontelle (Rossif Sutherland), and André Marek (Gerard Butler), to excavate the ruins of Castlegard in France. The village was razed in 1357 during the Hundred Years War, a significant event that inspired a French victory, spurred by the hanging of Lady Claire (Anna Friel), the sister of the valiant Arnaud de Cervole. While conducting their research, Chris's attention is more focused on Kate than on the dig, until André reveals a sarcophagus containing a French knight with a unique anomaly—his ear has been lopped off.
Suspecting foul play from ITC, Prof. Johnston decides to visit their headquarters in New Mexico. A few days later, without any updates, the team stumbles upon a 600-year-old parchment begging for help, along with one of the professor’s lost bifocal lenses. When Chris contacts ITC for information about his father, he is assured that everything will be explained in New Mexico. Once there, ITC president Robert Doniger (David Thewlis) and vice-president Steven Kramer (Matt Craven) reveal a shocking secret: they possess a time travel device powered by a wormhole, enabling travel back to Castlegard in 1357. Realizing that his father is trapped in the past, Chris, Kate, and André convince François to join them as he is the only fluent French speaker among them. They are accompanied by three marines to ensure security, unbeknownst to them that one marine has a covert mission to eliminate someone.
The team arrives in a perilous scenario, facing a rushing river and aggressive English riders, as warned by the French fugitive, Lady Claire. During this chaotic entry, two marines meet their demise. One marine, as he lays dying, activates his marker—a device that could return them to the present—but not before a forbidden grenade causes severe damage to their time machine. In a tense reunion, Claire reconnects with her brother Arnaud (Lambert Wilson) during their capture by the English forces, particularly the ruthless Lord Oliver de Vannes (Michael Sheen), who orders the execution of François under the guise of espionage.
As the group finds themselves imprisoned in an attic alongside Prof. Johnston, Kate, being the lightest and most agile, cunningly escapes through the roof to liberate her friends. They prepare for a confrontation against their captors, one that becomes increasingly violent. They realize that saving themselves may inevitably lead to regret, particularly when Sir William DeKere (Marton Csokas) reveals his identity as a former ITC insider, now severely changed from the toll taken by time travel.
DeKere, armed with their markers, causes chaos, leaving the group scrambling for survival as they gather to activate their last chance for return. With the markers’ energy waning, it becomes crucial for them to remain united in a clear space for their escape. During this time-traveling adventure, André finds a romantic connection with Claire, while Chris and Kate spark their own budding romance. As Kate leads Chris and Arnaud’s forces to a secret passage under La Roque Castle, their intervention leads to a twist in the battle, swinging the odds in favor of the French.
In a final clash, Lord Oliver captures Claire, but Arnaud avenges his sister by killing Oliver, and André, faced with DeKere, struggles to embrace his true identity as the knight from the sarcophagus. With newfound courage, he defeats DeKere, allowing the rest to gather for their return to the present.
Back in modern times, Josh and Kramer work to repair the damaged time machine, despite Doniger's indifference to the cost of human lives for the sake of scientific progress. As the crew successfully returns, Doniger, eager to continue his experiments, recklessly attempts to enter the transmit area without a marker and is swiftly met with a fatal fate by a charging knight. Later, at the ruins of Castlegard, the archaeologists find solace in reading the sarcophagus inscription created by André and Claire, which speaks of a life filled with joy and the legacy of their three children: Christophe, Katherine, and François.
