Valleby’s police station is threatened with closure. When the venerable Granat auction house comes to visit, Jerry and Maya see their chance to show Valleby what its police authority is made of, thereby saving the police station. With five newly installed alarms, and round-the-clock surveillance of the auctions house’s priceless treasures, JerryMayas’s Detective Agency is going to make sure that nothing goes wrong. But their security plans are thrown out of whack when the notorious thief Art Shadow turns out to be on the loose. She’s known for being able to steal anything, and when the Valleby citizens’ prized possessions start disappearing one by one, the Art Shadow immediately becomes the prime suspect. Gabriella, the police chief from the neighbouring town, is brought in and catches the thief all by herself. Valleby’s police chief is fired, and the services of JerryMayas’s Detective Agency are no longer required. But when things continue to disappear, even though the Art Shadow is now safely behind bars, Jerry and Maya realise that the wrong thief has been caught, and now have to solve the mystery themselves.