In Egypt, Yousef Bey (John Carradine) journeys to a remote temple in answer to a summons from Andoheb, High Priest of Arkham (George Zucco). Andoheb has called him to undertake a mission for the priesthood, but first refreshes his memory on the story of Kharis (Lon Chaney Jr.).
3000 years ago, Kharis and the Princess Ananka were involved in a forbidden love affair. The princess died and because of her sin, her soul was cursed forever. Kharis was buried alive along with her. 30 years prior to the present time, an expedition located and entered the tomb of Ananka. They fought and escaped from the guardian of the tomb-the living Mummy of Kharis. The archaeologists brought Ananka's body back to the Scripps Museum in the United States. However, the priests of Arkham sent Kharis to seek vengeance against the defilers of the tomb. Every person from the expedition that entered the tomb died at the Mummy's hands. Kharis was believed to be destroyed in a fire at the residence of Stephen Banning (Dick Foran), leader of the expedition.
The same story is being told to a group of skeptical college students in Mapleton Massachusetts. Professor Norman (Frank Reicher) expands on the story, explaining how Kharis was kept alive by the fluid of the leaves of the Tana bush, a plant that has long been extinct. In fact, Professor Norman himself was entrusted with a quantity of the tana leaves that investigators found in order to conduct scientific research.
After the lecture ends, one of the students, Tom Hervey (Robert Lowery), meets up with his girlfriend Amina Mansori (Ramsay Ames), a beautiful woman of Egyptian descent. Amina has been researching Egyptian history, which has brought on a strange, clouded feeling in her mind.
Back in Egypt, Andoheb informs Yousef Bey that Kharis still lives and that Yousef's mission is to retrieve Kharis and the body of Ananka and return them to their rightful resting place in Egypt. Yousef Bey then pledges his devotion:
"I swear by the mighty power of Amon-Ra, whose anger can shatter the world and by the dread horror of Set, that I shall never forsake my trust as a Priest of Arkham. Nor shall I rest until the Princess Ananka and Kharis are safe once more in the Hills of Arkham."
Andoheb explains that each night during the cycle of the full moon, Yousef Bey is to brew the fluid from 9 tana leaves and that wherever Kharis is, the Mummy will sense this and find the leaves. The moon is full in Mapleton as Professor Norman studies the hieroglyphics on the case of tana leaves. He has deciphered the message about brewing 9 tana leaves during the full moon and decides to do just that.
The battered, ragged form of Kharis now appears, his right arm is useless and held in a sling, his right eye burned and sealed shut. He senses the leaves brewing and begins instinctively heading toward them. On the way, he passes the home of Amina and his presence arouses her from a fitful sleep. She wanders outside in a trance-like state. She does not see Kharis, who is now some distance ahead, but somehow follows the same course. Kharis soon arrives at the home of Professor Norman and enters his study. Norman looks up and is shocked to see the Mummy shambling toward him with one arm extending, reaching for the Professor's throat. Soundlessly and brutally, Kharis strangles Norman and then drinks the fluid of the tana leaves.
Amina now approaches and Kharis senses her presence and heads back outside. From a concealed location, Amina sees Kharis, which snaps her out of her trance but also causes her to immediately faint. She falls to the ground with a strange birthmark now apparent on her wrist.
The next morning, Tom Hervey learns that Professor Norman was murdered and that Amina was found at the scene and detained. He immediately heads for the Professor's home. At the home, Sherriff Elwood (Harry Shannon) and the Coroner (Emmett Vogan) investigate the scene of the crime. They discover a strange mold around the Professor's throat; a sign they both know to mean that the Mummy stalks Mapleton again. The Sherrif questions Amina, who has developed a white streak in her hair overnight. Amina is dazed and confused, but Elwood continues to grill her until Hervey arrives. Tom unsuccessfully tries to provide an alibi for Amina and the Sherrif maintains his suspicions in her involvement. Elwood finally dismisses the pair but gives Amina the obligatory warning not to leave the area. Tom takes her home to comfort her but is himself disturbed by the changes in her appearance.
Panic begins to grip the town as newspaper reports of the killing highlight the involvement of the Egyptian woman and the speculation that the Mummy has returned. Townspeople are warned to stay indoors at night and all able-bodied men are asked to help patrol the streets after dark.
Weeks later Yousef Bey, who has arrived in Mapleton, calls on Amon-Ra to aid him in his quest and begins to brew the sacred fluid of the tana leaves to summon Kharis. Kharis, indeed, senses the leaves and heads toward them. Tom and Amina are out for a drive that same night. Amina is finally beginning to return to normal, laughing and no longer hearing voices or suffering from fevers and chills. Unseen, Kharis passes near them, causing Amina's mood to suddenly change and for her to feel an unnatural cold. Despite Tom professing his love for her, Amina cannot shake the strange feeling and asks to be taken home.
Kharis continues on his quest for the tana leaves, passing through the Evans farm. The family dog is startled by his presence and begins to bark incessantly. Mr. Evans (Eddy Waller), armed with a shot gun, comes out of the house to investigate and is surprised by Kharis. Evans fires his weapon at the Mummy to no avail. Kharis swiftly murders the man and busts through the wall of the barn, undeterred from his quest. The Sherrif soon arrives on the scene and, recognizing the evidence of Kharis, plans to organize a search party. Kharis, meanwhile, finds Yousef Bey and consumes the life-giving tana leaves. Yousef thanks his god for his success thus far and re-pledges his devotion to their mission.
The next day, at the Scripps Museum, Yousef Bey lags behind a tour group viewing the Mummy of Ananka. He asks Amon-Ra for forgiveness for allowing these heretics to view the sacred remains of the Princess. After closing time, Yousef emerges from a hiding place as Kharis breaks into the museum. The two are excited with anticipation as their mission appears near an end. Yousef encourages Kharis by telling him the Tombs of Arkham are waiting to claim their own. Kharis attempts to touch the mummified body, but it disintegrates under the wrapping as his hand approaches. At that same moment, Amina, miles away in her home, awakens screaming. Yousef Bey realizes that Ananka's soul has been re-incarnated into another form. Kharis is enraged and begins destroying the exhibit, attracting the museum security guard (Oscar O'Shea). The guard enters, not seeing Kharis, and draws his gun on the High Priest. The Mummy exits the shadows and moves toward the horrified man. He fires several shot at point blank range, but the Mummy is not even slowed. Kharis crushes his throat as Yousef Bey looks on with glee. Yousef Bey realizes that the two must still fulfill their mission and find the new vessel holding Ananka's soul and return it to Egypt.
Tom Hervey visits Sherrif Elwood, seeking permission to take Amina away to New York to restore her health. Elwood says he is not trying to implicate Amina in the murders, but denies Hervey's request. At the Scripps Museum, the body of the guard is found and the tell-tale marks of mold identify the killer as Kharis. Police Inspector Walgreen (Barton MacLane) and Dr. Ayad (Lester Sharpe) from the museum are bewildered as to how Ananka's body has disappeared without disturbing the wrappings. Dr. Ayad translates the markings on the tomb for the Inspector:
"Herein rests the body of Ananka, third daughter of Amenophis. In the Hills of Arkham shall Ananka lie in the tomb appointed for her and in none other, because freed from its resting place, her soul may find reincarnation in another form, again to seek out its salvation on earth."
The Inspector is skeptical but asks the Doctor to accompany him to Mapleton to investigate further. Upon arriving, they seek out the assistance of Sherrif Elwood and the group convenes at the home of the late Professor Norman. Dr. Ayad translates most of the markings on the cask of tana leaves and Inspector Walgreen decides to use the leaves to attract and capture Kharis. Since bullets are useless against the Mummy, the plan is to build a pit to confine the creature until a way to deal with him can be found.
Amina is still unable to shake the haunted feelings that torture her. Tom, disregarding the Sherrif's warnings, asks Amina to elope with him to New York. Amina tries to discourage him but Tom is insistent. She agrees and the two make plans to leave early the next morning. Tom leaves Amina in the care of her guardian, Mrs. Blake (Dorothy Vaughan) and Tom's faithful dog, Peanuts.
Yousef Bey calls upon Amon Ra to lead him to the new home of Ananka's soul. Just then, a window blows open and Yousef sees the moon, which resembles the scene on the ceremonial amulet he wears as a Priest of Arkham. Knowing that this is a sign from the gods, he sends Kharis in that direction to find Ananka.
Using the widowed Mrs. Norman's (Claire Whitney) recollections as a guide, Inspector Walgreen now begins to bait his trap by burning nine tana leaves. Kharis immediately senses this, and heads toward the Norman home--a course that takes him directly past Amina's home. Amina is awakened by his approach and hypnotically wanders into the yard. Kharis sees her and recognizes her as the carrier of Ananka's soul. Amina faints as Kharis picks her up and takes her away.
The abduction is witnessed by Mrs. Blake, who phones Tom to alert him. Tom immediately sets out in pursuit. Mrs. Blake, then heads to the Norman house and tells her story to Inspector Walgreen, Sherrif Elwood and a large group of volunteers. A frantic chase now ensues. Kharis and Amina are headed for the old mill where Yousef Bey waits. Tom, led by Peanuts, is hot on their trail, while further back is the mob. Kharis arrives at the mill and presents Amina to Yousef Bey. Bey recognizes the birthmark on her wrist as the symbol of the Priests of Arkham. Amina awakens and the Priest informs her that she is, indeed, the reincarnation of Ananka. Amina denies this but Yousef Bey warns her:
"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."
Yousef Bey now begins to admire Amina's beauty and feels the pangs of temptation to keep her alive as his bride. His devotion begins to falter as they are thousands of miles from the Tombs of Arkham and Ananka is thousands of years from her sin. He calls upon Isis to protect him in his hour of temptation, but his prayers go unanswered. He decides to use the tana leaves to keep Amina young and beautiful forever. The two will drink the fluid and become immortal and never return to Egypt. This enrages Kharis and before Yousef Bey can give her the fluid, the Mummy knocks the cup away. He exacts his vengeance on the Priest with a killing blow, delivered with enough force to fling Yousef's lifeless body through the window and onto the ground below.
Tom Hervey now arrives and witnesses the death of the Priest. He rushes up the stairs to the mill but is met by Kharis. A struggle ensues and Tom is quickly overwhelmed. Kharis returns to retrieve Amina, whose hair is now completely white. The mob now approaches and they revive Tom, as Kharis attempts to escape with Amina. Once again led by Peanuts, the mob pursues the Mummy and his Princess into the neary swamps. In Kharis's arms, Amina/Ananka is now aging rapidly. They are chased deeper and deeper into the swamps and now begin to sink into the bog. Tom's last anguished sight of Amina is that of a 3,000 year old Egyptian Princess. Kharis and Ananka disappear under the water, united in death.