Egyptian Jousef Bey (John Carradine), a High Priest of Arkam, is sent to the U.S. to retrieve the mummified corpse of the 3,000 year old princess Ananka that is currently on display at the Scripp's Museum near Mapleton and to find and bring back to Egypt her protector, the living mummy Kharis (Lon Chaney Jr.), who somehow manages to exist after being burned to cinders in a fire. Meanwhile, Kharis' presence is somehow having a strange effect on a young Egyptian girl, Amina Mansori (Ramsay Ames), whose boyfriend Tom Hervey (Robert Lowery) is studying under prominent Egyptology Professor Matthew Norman (Frank Reicher), who is currently trying to unlock the secret of the Kharis' longevity.

The Mummy's Ghost is the fourth movie in Universal Studio's Mummy series, preceded by The Mummy (1932) (1932), The Mummy's Hand (1940) (1940), and The Mummy's Tomb (1942) (1942) and followed by The Mummy's Curse (1944) (1944). The movie was filmed from a screenplay written by American screenwriters Griffin Jay, Henry Sucher, and Brenda Weisberg.

The Mummy's Tomb took place 30 years after archeologist Steve Banning discovered Ananka's tomb (in The Mummy's Hand. In The Mummy's Ghost, Andoheb (George Zucco) also tells Bey that Banning's find took place 30 years ago. The conclusion is that The Mummy's Ghost follows The Mummy's Tomb by a matter of months, maybe even weeks.

Uncertain. It's mentioned only at the start of the movie that Kharis perished in the fire at the end of The Mummy's Tomb. The implication is that Kharis survived the fire (although no mention of a body is made), that he was somehow reanimated, or that the re-appearance of the Kharis mummy is a ghost. The movie goes no further in answering that question.

Kharis carries the unconscious Amina to the old mill where Bey waits for them. When she awakens, Bey explains to her that she is the Princess Ananka, daughter of Pharoah Aminifis. When Amina tells Bey that he's mad, he warns: For those who defy the will of the ancient gods, a cruel and violent death shall be their fate, never shall they find rest unto eternity. He admires Amina's baauty and begins to argue with himself over keeping his promise of devotion to his duty or taking Amina as his bride and making them both immortal. He decides on the latter and tells Amina, her hair now turned completely white, of his plan to not return to Egypt in order to be together forever. Kharis overhears Bey's plan and is enraged. Before Bey can give the tana fluid to Amina to drink, Khanis knocks the cup away and attacks Bey, hitting him hard enough to knock him out a window. Bey's body falls to the ground below just as Tom and his dog Peanuts arrive, followed not far behind by the crowd of volunteers. Tom climbs up to the mill shack, but Kharis knocks him back down. While the mob begins to ascend to the shack, Kharis carries Amina down another ladder and heads toward the swamp. By the time Kharis reaches the swamp, Amina has aged into an old hag. Kharis carries her deep into the swamp, and they both disappear under the water. In the final scene, the camera pans over the still swamp, and the audience is reminded of Bey's warning: The fate of those who defy the will of the ancient gods shall be a cruel and violent death.