In ancient Egypt, Tutankhamun, the boy pharaoh who is said to have died young, was actually sent by the sun god Ra to protect the people from the demon Set, who escaped from the Underworld and caused chaos in Egypt. With Ra's blessing, Tutankhamun manages to overcome Set and keep the demon under control by breaking an emerald tablet into four pieces and sending the pieces to the far corners of the world. However, the tradeoff is that he must remain in the Underworld.
In 1922, Danny Freemont, an archaeologist, is searching for the last piece of the tablet, which is believed to be in Tutankhamun's tomb. He found the first three pieces earlier, but they were seized from him by his rival, Morgan Sinclair. Sinclair is in the service of the Hellfire Council, a secret committee formed by a group of influential men from around the world, and they wish to use the tablet's powers to achieve their goal of world domination. Sinclair uses the Hellfire Council's influence to damage Freemont's reputation and cause him to lose his job.
Freemont is undaunted by the challenges he face. With help from his buddies, he manages to convince several others, including a sceptical Egyptologist named Azelia Barakat, to join him in his quest to find Tutankhamun's tomb and the last piece of the tablet. Despite their efforts, the last piece still falls into the hands of Sinclair and the Hellfire Council. Sinclair assembles all four pieces in the tomb, absorbs the tablet's powers, and unleashes Set and the demons of the Underworld. He also gets rid of the Hellfire Council's members later.
Freemont and his companions manage to hold off Sinclair and other enemies and make their way back to the tomb, where they open a portal to the Underworld. In the Underworld, Freemont and his friends succeed in finding and freeing Tutankhamun, but the boy pharaoh is too weak to help them. Meanwhile, Sinclair is absorbed by Set, who then attacks Freemont. At the critical moment, Barakat prays to Ra to empower Tutankhamun and her wish is granted. Tutankhamun springs to life, fights Set, and brings Freemont and Barakat (the only two survivors) safely out of the Underworld. Before leaving, Tutankhamun thanks them and tells them that "all things are as they should have been". Freemont and Barakat do not understand what he means and they leave the tomb just before it closes by itself.
Back in the streets of Cairo, Freemont and Barakat finally understand what Tutankhamun meant when they see that all the negative events which happened earlier have been reversed: their dead companions are alive and well; Barakat's fiancé is happily married to another woman and has a family. Freemont chances upon Howard Carter and gives him the map to Tutankhamun's tomb; Carter becomes world famous for his "discovery". At the end, in the Museum of Antiquities, Freemont proposes to Barakat and she agrees to marry him.