On holidays in Hong Kong, Mrs. Ma (Sylvia Chang) gives birth to identical twins. A criminal in the same hospital attempts to escape, taking one of the twins hostage. The child is lost during the confusion, and Mr. (James Wong) and Mrs. Ma return to New York with one child. Years later, Ma Yau (Jackie Chan) is a famous conductor and pianist, unaware that his twin brother 'Boomer' is a mechanic/race car driver/bodyguard in Hong Kong. When Ma Yau travels to Hong Kong to give a concert, the twins get caught up in each other's business, about which they are anything but experts.