Two Americans, Bud Abbott and Lou Costello, who are stranded in Cairo, Egypt, happen to overhear Dr. Gustav Zoomer (Kurt Katch) discussing the mummy Klaris, the guardian of the Tomb of Princess Ara.
Apparently the mummy has a sacred medallion that shows where the treasure of Princess Ara can be found. The followers of Klaris, led by Semu (Richard Deacon), overhear the conversation along with Madame Rontru (Marie Windsor), a businesswoman interested in stealing the treasure of Princess Ara.
Abbott and Costello go to the doctor’s house to apply for the position to accompany the mummy back to America. However, two of Semu’s men, Iben (Mel Welles) and Hetsut (Richard Karlan), murder the doctor and steal the mummy just before Abbott and Costello arrive.
The medallion has been left behind, though, and is found by Abbott and Costello, who attempt to sell it. Rontru offers them $100, but Abbott suspects it is worth much more and asks for $5,000, which Rontru agrees to pay. She tells them to meet her at the Cairo Café, where Abbott and Costello learn from a waiter that the medallion is cursed.
They frantically try to give it to one another (the Slipping the Mickey routine from The Naughty Nineties), until it winds up in Costello’s hamburger and he swallows it.
Rontru arrives and drags them to a doctor’s office to get a look at the medallion under a fluoroscope. However, she cannot read the medallion’s inscribed instructions, which are in hieroglyphics. Semu arrives, claiming to be an archaeologist, and offers to guide them all to the tomb. Meanwhile, Semu’s followers have returned life to Klaris.
They arrive at the tomb, where Costello learns of Semu’s plans to murder them all. Rontru captures Semu, and one of her men, Charlie (Michael Ansara), disguises himself as a mummy and enters the temple. Abbott follows suit by disguising himself as a mummy, and he and Costello rescue Semu.
Eventually all three mummies are in the same place at the same time, and the dynamite that Rontru intends to use to dig up the treasure detonates, killing Klaris and revealing the treasure. Abbott and Costello convince Semu to turn the temple into a nightclub to preserve the legend of Klaris and the three criminals who wanted to steal the treasure are presumably arrested.