In present-day London, construction workers discover the tomb of a crusader knight who was buried with an Egyptian ruby in 1127 AD. In Iraq, U.S. Army soldiers Sergeant Nick Morton and Corporal Chris Vail discover the tomb of Princess Ahmanet, who attempted to summon the deity Set; she was caught and mummified alive. They and archaeologist Jenny Halsey fly Ahmanet's sarcophagus to Britain.
During the flight, Chris is possessed by Ahmanet through a spider that had bitten him in the tomb, attacks the others, and is killed by Nick. Many crows attack the plane, causing it to crash, killing everyone except Jenny, who escapes by a parachute Nick gives her.
Nick returns to life in a morgue. Chris's ghost appears and tells him he has been cursed by Ahmanet, who seeks to use him as a vessel for Set. Ahmanet's mummy escapes from the sarcophagus and begins feeding on people to regenerate her body; she recovers the Dagger of Set and turns workers into zombies which trap Nick and Jenny.
Soldiers appear and subdue Ahmanet. Their leader, Dr. Henry Jekyll, explains that Jenny is an agent of Prodigium, a secret society dedicated to hunting supernatural threats. Nick and Jenny then discover Dr. Jekyll intends to allow Set to possess Nick completely in the belief that this will render Set vulnerable.
Jekyll then transforms into Edward Hyde; Nick stops him with a serum Jekyll invented to prevent him from undergoing this transformation, and he and Jenny escape. Ahmanet finds them, regains the dagger, summons many crusader zombies, and creates a large sandstorm in London; Nick and Jenny escape her. The crusader zombies kill the Prodigium soldiers; Ahmanet recovers the ruby, combining it with the dagger in order to free Set.
Guided by Chris's ghost, Nick and Jenny flee into the London Underground. They are attacked by Ahmanet's minions; Ahmanet captures Jenny and drowns her. Nick tries to attack Ahmanet and ends up sustaining injuries. He lets Ahmanet embrace him, a ruse to let him steal the dagger and shatter the ruby. He strikes the pommel of the dagger against the floor, cracking the ruby. As he is about to shatter the ruby for good, Ahmanet reminds him that the dagger could grant him the ability to reverse death. He stabs himself instead and because the ruby is cracked, becomes partially possessed by Set. He goes to Ahmanet, but, upon remembering both Jenny and Dr. Jekyll's words, turns on her and uses Set's powers to suck out her lifeforce and resurrect Jenny. Knowing the Prodigium are coming for him, he says goodbye to Jenny before he vanishes.
As Ahmanet is sealed back in her tomb, Jenny and Dr. Jekyll debate whether Nick, now fused with Set, will use his powers for good. Nick returns to the desert and resurrects Chris. The two then go on a new adventure.
