In Dordogne Valley of France archeologist professor Edward Johnston (Billy Connolly) and his students André Marek (Gerard Butler), Kate Ericson (Frances O'Connor), Josh Stern (Ethan Embry) and François Dontelle (Rossif Sutherland) are excavating a site near the ruins of Castlegard. Johnston has made major strides in unearthing not only the village, but also a monastery and structures from the surrounding of Castlegard, and the 14th-century castle La Roque. During the Hundred Years War , in 1357, the village had been burned, and massive battle occurred at nearby the La Roque. Even though the English were strong, the French were able to defeat them. Lady Claire (Anna Friel), the sister of Arnaut de Cervole (Lambert Wilson) was hanged by the English Lord Oliver de Vannes (Michael Sheen) at the castle’s battlements in full view of the French. This enraged the French so much that they won the battle against the English.

Johnston and his students live for the past. Johnston's son Chris (Paul Walker) does not share this passion but he visits his father from time to time mainly to be with Kate whom he has a crush on. Suspicious of the dig's benefactor, International Technology Corporation (ITC) and the man who runs it, Robert Doniger (David Thewlis), Professor Johnston heads to ITC headquarters in New Mexico to get some answers, and while he's away, Kate and Marek found a chamber that has been sealed for more than 600 years. They descend into the unstable room, and just before a near disastrous cave-in, they make two startling discoveries: a bifocal lens, which couldn't have been invented before the chamber was sealed, and even more intriguing, a handwritten plea for help dated April 2, 1357. Examination shows that the lens belong to Professor Jonhston’s glasses, as well the writing on the document, but it was wrote with a 600 years old’s ink. Confused by this, Chris calls his father at the sponsors’ but is told he is no longer there. They do, however, invite him and the students to their base in New Mexico.

Determined to solve the mystery, the students head for ITC headquarters, where they are stunned to learn of Doniger's new invention, a machine that can actually transmit three-dimensional objects through space. Although he meant for the device to revolutionize shipping, Doniger inadvertently opened a wormhole that leads directly to the 14th century, and Professor Johnston, who had insisted on experiencing the discovery himself, is now trapped in 1357, near Castlegard, exactly on the spot where he was performing the excavations. As the students are the closest thing to experts on the locations and the time period, they are asked to go back in time and rescue him. They agree and are joined by some military personnel leads by Frank Gordon (Neal McDonough). Everyone has a marker, to call for the return.

They arrive in the middle of the battle between the English and the French. The military personnel is killed, except Frank, but one of them come back in the present using is marker, and a grenade as well. Back in the present day, the laboratory where the time travelers started off is severely damaged by the grenade explosion and they cannot be brought back without rebuilding the time device.

André is caught up in the battle and saves the Lady Claire by slaying a knight who was attacking her. She brings them to the safety of her village but they are betrayed and captured by the English. They are seen as spies, François is killed, ‘cause he’s french, and the others are reunited in prison with the Professor who had been similarly taken. The group devises an escape plan but they marker are not working. Of course they didn’t know about the explosion in the time machine, that Stern and Doniger’s assistant have to fix in less than 6 hours. But they do know exactly what will happen during the course of the day and will try to use that to their advantage. Unfortunately they do not take into account that their presence there has already started changing the timeline.  

Andrè save Lady Claire’s life, escaping with her, and taking her back to his brother Arnault. Meanwhile Professor Johnston and Frank are caught by the English, as well as Marek and Lady Claire. An ITC scientist, trapped in the past, and now the english Lord Deker, kills Frank, but Professor and Marek are lead to the castle, where Lord Oliver ask them to make for him the famous Greek fire. Chris and Kate reach the near monastery, and they found the chamber where Kate was working in. They think to have found the secret way that leads to the castle, and they advise Lord Arnult. Battle start, Lady Claire is hanged at the castle, but the greek fire created by Professor and Marek seems to change the battle’s history. But French use the secret way and they enter into the castle’s court, fighting from inside the English. Kate and Chris reunite with Professor and Marek, but are involved in the battle. They found a way out, but André chooses to remain behind to stay with his new found love, Lady Claire. In the present time the machine is fixed and the surviving travelers can return home.

Jerry Goldsmith had scored and recorded the music for the film in late December 2002. However, Richard Donner decided to do a partial reedit of the film, which forced Goldsmith to come in and fix his score in March 2003. When Donner decided to recut the film once more, he realized that Jerry's score was not what he was looking for, describing it as "beautiful, but not loud enough." Goldsmith was offered the chance to redo his score from scratch, but declined because he was fed up with the movie. Donner then called upon Brian Tyler to do the score for him. Goldsmith's score was released by Varese Sarabande Records in 2004.

The battle of Castlegard never took place. But the crew of the movie visited various European castles from around that period (the late 1300s) to make the castles and towns look as realistic as possible. However, there are similarities between besieger's strategy in the film and the actual historical event of the siege of Château-Gaillard in France in 1204.

A group of medieval re-enactors were used for soldiers in battle sequences.

The film was originally set to open in April 2003, but due to poor test screenings Sherry Lansing the head of Paramount Pictures at the time ordered that the film be re edited in the process losing a crucial subplot that is touched on in the final version involving David Thewlis' character and also Jerry Goldsmith's score was replaced because he was not able to return to the scoring stage due to health reasons.

Frank Cappello wrote an uncredited draft of the script and his name appeared on a few of the original teaser posters and trailers along side credited writers George Nolfi and Jeff Maguire.

The battle sequences used medieval reenactors.

Greek fire is a forgotten secret weapon, and no one knows the original formula used by the Greeks and the Byzantine Empire.

The character Robert Doniger was named after a teacher of Crichton's daughter, Harriet Jacobs Doniger.

Paul Walker and Lambert Wilson recently work again togheter in the drama The Lazarus Project (2008)