Rosie finds a mosaic in a passageway in Great-aunt Rosamund's old castle. When she sees a sad-looking girl in a toga on the mosaic, Rosie is convinced that she's found another little princess. She's right! Before she knows it, Rosie finds herself in a beautiful garden by a temple in ancient Rome and meets the daugher of the emperor of Rome - Princess Isadora.
Princess Isadora is sad because the goddess Juventas has put a curse on all the young boys because Isadora's brother, Marcus, forgot to leave a gold coin as an offering to the goddess after his coming of age ceremony. Juventas has made all the young boys of Rome age, so they are now all old men and they will die soon if Isadora can't do anything to help them!
Rosie and Isadora go to the forest to ask Acis the faun for advice. He tells them to ask Cupid, the god of love for help. If Cupid hits Juventas with one of his arrows, the stony heart of Juventas will melt and she will take back the curse. Can the girls get Cupid to help them before it's too late? And will they manage to escape the wrath of the Harpies who are doing their best to stop them?