A collection of artifacts from an archeological dig in Egypt are brought to the famous Louvre museum in Paris, and while experts are using a laser scanning device to determine the age of a sarcophagus, a ghostly spirit escapes and makes its way into the museum's electrical system. Museum curator Faussier (Jean-Francois Balmer) brings in a noted Egyptologist, Glenda Spencer, to examine the findings, and she announces that the mummy inside the coffin was actually the evil spirit Belphegor. Lisa, who lives across the street from the museum, follows her runaway cat into the museum after closing time, where she is accidentally given a shock that sends the stray spirit into her body. Soon, Lisa is disguising herself as Belphegor and making off with the rare Egyptian treasures on display at the museum, convinced that they are rightfully hers. When "Belphegor" proves more than a match for the Louvre's security forces, renowned detective Verlac is brought out of retirement to find out why the museum's Egyptian collection has been shrinking.