Ibbotson, a Britisher, is known here primarily for her juveniles; and this bouncy period romance (Vienna, 1920s) has a streak of happy Study Hall silliness--which levitates the love-story of a Princess and a Newcastle foundling well above its cloying givens. The penniless foundling, Guy Fame, has become an international trader of whopping wealth; and he's about to buy the castle Pfaffenstein for the woman he intends to marry--Nerine Croft, a narcissistic beauty of ""breathtaking perfection."" But before he can purchase Pfaffenstein, Guy needs the consent of the near-penniless absent owner, Princess Theresa-Maria--known to her ancient aunts as ""Putzerl,"" known as the waif ""Tessa"" in Vienna, where she's an overworked gofer for a talented, eccentric opera company. Eventually all the principals--including Tessa's dim suitor, Prince Maximilian of Spittau--will meet at the castle, where the opera company is to perform Mozart's Flute. Tessa's Princess-dom is revealed to the stunned company; opera-loving Guy is wowed by the Mozart; the Princess is smitten with Guy--who's still attached to secret opera-hater Nerine. So, silent and suffering, the Princess returns to the Company, using her new wealth (from the castle sale) to support a majestic atonal work called FrieassÃ‰e (scored for 15 mandolins and 37 percussion instruments). And the correct pairing won't be accomplished till after the arrival of Guy's beloved Geordie mum, some prime acting by the Princess, and an operatic heist. With pretty scenery and a bundle of volatile, dippy performers and aristocrats: good, simpleminded fun.