In 1921, an archaeological expedition led by Sir Joseph Whemple (Arthur Byron) finds the mummy of ancient Egyptian priest called Imhotep (Boris Karloff). When an inspection of the mummy by Whemple's friend Dr. Muller (Edward Van Sloan) reveals that the viscera were not removed, Muller deduces that Imhotep was buried alive for sacrilege. Imhotep had been mummified alive for attempting to resurrect his forbidden lover, the princess Ankh-es-en-amon. Despite Muller's warning, Sir Joseph's assistant Ralph Norton (Bramwell Fletcher) reads aloud an ancient life-giving scroll the Scroll of Thoth. Imhotep escapes from the archaeologists, taking the Scroll of Thoth, and prowls Cairo seeking the modern reincarnation of Ankh-es-en-amon.
10 years later, Imhotep is masquerading as a modern Egyptian named Ardath Bey. He calls upon Sir Joseph's son, Frank (David Manners) and Prof. Pearson (Leonard Mudie). He shows them where to dig to find Ankh-es-en-amon's tomb. The archaeologists find the tomb, give the mummy and the treasures to the Cairo Museum, and thank Ardath Bey for the information.
Imhotep encounters Helen Grosvenor (Zita Johann), a woman bearing a striking resemblance to the Princess. Believing her to be Ankh-es-en-amon's reincarnation, he attempts to kill her, with the intention of mummifying her, resurrecting her, and making her his bride. She is saved when she remembers her past life and prays to the goddess Isis to save her. The statue of Isis raises its arm and emits a beam of light that sets the Scroll of Thoth on fire. This breaks the spell that had given Imhotep his immortality, causing him to age rapidly and then crumble to dust. At the urging of Dr. Muller, Frank calls Helen back to the world of the living while the Scroll of Thoth continues to burn.
