At an Egyptian café, bumbling adventurers Bud Abbott and Lou Costello overhear Dr. Gustav Zoomer explain to a journalist that he has just discovered the mummy of Klaris, the Prince of Evil, whose sarcophagus contains a clue leading to the long-lost tomb of the Princess Ara. Two other interested parties also listen closely to Zoomer: Hetsut and Iben, henchmen of Semu, the leader of Klaris' cult followers, who are eager to steal back their sacred god; and Rontru, a fortune-hunter who plots with her helpers, Charlie and Josef, to kill Dr. Zoomer. Bud and Lou, wanting Zoomer to hire them to accompany the mummy back to America, visit the doctor's castle that evening, not realizing that Hetsut, Iben, Charlie and Josef are hiding there. Unable to locate Zoomer, Bud and Lou search the rooms separately, and although Lou repeatedly finds evidence that other people are there, the clues disappear as soon as Bud arrives. Meanwhile, Hetsut and Iben find the mummy and sequester him in the basement, where they realize that his medallion, which contains the clue to Ara's location, is missing. Upstairs, Bud instructs Lou to hide in different rooms and photograph anyone he sees. While hiding, Lou repeatedly stumbles onto Zoomer's dead body, but each time he brings Bud back to view it, Hetsut has removed it temporarily, causing Bud to scoff at him. Finally, Lou enters Zoomer's office, where he inadvertently overhears a recording of Zoomer revealing that the medallion is in the grandfather clock. Bud finds Lou and has him photograph Zoomer's body, so they can anonymously send the pictures to the police. On their way out, Charlie and Josef overhear them discussing their plans and assume they killed Zoomer. Meanwhile, Hetsut and Iben sneak Klaris to his tomb, where Semu feeds the stirring mummy and then threatens his men to find the medallion or die. The next day, Bud sees in the newspaper that Lou photographed him along with Zoomer's body, and is now wanted for the doctor's murder. He and Lou flee the police, disguising themselves as snake charmers to escape detection. Hoping to catch the real murderer, they return to Zoomer's at the same time as Rontru, Charlie and Josef, who are hiding in the castle and attack as soon as Lou uncovers the medallion in the clock. When the police then appear outside, Bud and Lou fight everyone off, escaping through a window. They race to a pawnshop to hock the medallion, but when the owner sees it, he screams that it is cursed. Rontru then appears and offers to buy it from them that night. They meet at the café, where Bud and Lou pass the medallion back and forth, each one hoping to foist the curse off onto the other. When Bud places it in Lou's burger, Lou swallows it, prompting Rontru to abduct them and take them to a doctor's office, where she x-rays Lou's stomach. Semu has followed them, and when he pretends to be an amateur archeologist, she hires him to lead them to Klaris' tomb, using hieroglyphics inscribed in the medallion and exposed in the x-ray as a guide. The next day, as they reach the mouth of the tomb, Semu disappears into a secret entrance and organizes his followers inside. Lou accidentally discovers the hidden door by falling through it, then stumbles into several giant prehistoric beings before being pursued by an awakened Klaris. As Bud joins Lou inside, Klaris catches Lou and, by shaking him upside down, retrieves the medallion. Outside, Rontru sneaks up on Semu and ties him up, and then disguises Charlie as a mummy, enters the tomb and knocks out Klaris. After she replaces his body with Charlie's and buries Klaris outside, Bud and Lou find Charlie and, thinking he is Klaris, knock him out and unknowingly bury him in the same hole as the real mummy. Finding leftover bandages, Bud wraps himself up like a mummy, after which he and Lou discover Semu and accidentally push the still-bound man into the tomb via another hidden door. Just as Bud and Lou re-enter the tomb, both Charlie and Klaris wake and follow them. Soon after, the three mummies frighten one another in the labyrinthine passages. Semu's followers find him and everyone enters the tomb in time to see Klaris attack and kill Rontru. Josef tosses some dynamite he has been holding to Lou, who throws it to Klaris just as it explodes. When the dust clears, the mummy is dead and Ara's sarcophagus has been exposed. To cheer up Semu, Bud proposes a new way to honor the Prince, and soon after, Café Klaris opens to great success.