While she's getting ready for an Egyptian-themed party at her friend's house, Rosie spots a papyrus painting on a wall in Great-aunt Rosamund's castle. It's of a young girl in a beautiful white dress, wearing lots of gold jewellery - it must be another little princess! As soon as she says hello, Rosie is whisked away in a warm breeze heavy with the smell of perfume and sparkling with grains of fine gold sand. Rosie meets Princess Aisha, who is very sad because her elder sister Nafretiri is ill. The priests say that an evil spirit is trying to take Nafretiri to the Underworld. There's only one way that she might be saved - an ancient legend says that a magic potion lies hidden in a secret cavern but only the mighty Sphinx knows where it is. And the Sphinx itself may not even exist! But Rosie and Aisha are determined to do something to help so they set off up the Nile in search of the potion that could save Nafretiri's life.