In Thebes, Egypt, 1290 BC, the high priest Imhotep has an affair with Pharaoh Seti I's mistress Anck-su-namun. The pair kills the Pharaoh after he discovers their transgressions. When the Pharaoh's bodyguards, the Medjai, arrive, Anck-su-namun takes her own life while Imhotep flees. Imhotep and his priests steal her corpse and travel to *Hamunaptra, the City of the Dead***, to conduct a ritual to resurrect her, but the Medjai intervene. As punishment, Imhotep's priests are mummified alive, while Imhotep himself is cursed and buried alive with flesh-eating scarabs. The Medjai are sworn to prevent Imhotep's return, as his resurrection would grant him immense power.
In 1926 AD Cairo, Kingdom of Egypt, Jonathan Carnahan presents his sister, Evelyn—a librarian and aspiring Egyptologist working for museum curator Dr. Terence Bey—with an intricate box and map that lead to Hamunaptra, the existence of which is now considered a myth along with the treasures believed to reside there. Jonathan reveals he stole the box from American adventurer and treasure hunter Rick O'Connell, who discovered the city while in the French Foreign Legion. Evelyn bargains with corrupt prison warden Gad Hassan to free an incarcerated Rick, in exchange for the American leading them to Hamunaptra where she hopes to find the golden Book of Amun-Ra.
The group soon finds themselves competing with another expedition, guided by Rick's cowardly fellow former Legionnaire Beni Gabor. They are repeatedly intercepted by Medjai, whose leader Ardeth Bay cautions them to leave. They ignore Bay's warnings; the two expeditions continue to dig through the ruins, resulting in numerous fatalities that include Hassan being killed by a scarab he inadvertently releases, as well as some workers who are melted by an acid trap that was intentionally left behind to deter future excavation. Both expeditions come into conflict over a statue which is believed to hold the golden book. Within the statue, however, the competing groups come across the Book of the Dead as well as five canopic jars, while Evelyn, Rick, and Jonathan accidentally discover Imhotep's mummified remains.
That night, Evelyn steals the Book of the Dead from the rival camp, accidentally awakening the mummified Imhotep when she reads the text contained within. Following a confrontation with the now resurrected Imhotep as well as the Medjai, the expeditions flee back to Cairo. Imhotep spares Beni's life to serve as a translator in exchange for treasure when the mummy realises that he can speak Hebrew.
Imhotep follows the expeditions back to Cairo and unleashes the biblical Plagues of Egypt. Those who discovered the canopic jars are hunted and killed by Imhotep, restoring his human form and strengthening his power. Attempting to find answers at the museum, the survivors instead find Ardeth being hosted by Dr. Bey, a fellow Medji. The group surmises that Imhotep desires to resurrect Anck-su-namun using Evelyn as a human sacrifice, and that the only way to stop him is to return to Hamunaptra and find the Book of Amun-Ra. The group are soon cornered by Imhotep and a now enthralled city populace, which results in Evelyn's capture while Dr. Bey sacrifices himself to allow the others to escape.
The remaining three convince Rick's old friend, Royal Air Force Captain Winston Havelock, to fly them to Hamunaptra. Imhotep conjures a magical sandstorm, crashing their plane and killing Havelock. After reaching the city on foot, Rick and Jonathan find the Book of Amun-Ra, while Ardeth is overwhelmed fighting Imhotep's resurrected servants. Jonathan and Rick reach the ritual chamber, where Rick frees Evelyn, and Jonathan uses the book to control the resurrected servants, leading to the death of a partially resurrected Anck-su-namun. Evelyn then uses the book to make Imhotep mortal, allowing Rick to fatally impale him; Imhotep decomposes back to his mummified form as he sinks into a pool of liquid, swearing vengeance in the process.
Meanwhile, Beni, who had been pilfering treasure from Hamunaptra, inadvertently triggers a booby-trap, causing the city to start collapsing. Rick, Evie and Jonathan escape while Beni is buried inside and devoured by scarabs. Outside, the group discover Ardeth survived, and he bids them farewell as they depart, unknowingly with some of Beni's plundered loot.
