Alexander and Emily travel to London to celebrate the 50th anniversary of the Queen's coronation. There is great excitement in Buckingham Palace, the big diamond from the crown has disappeared. The girl Holly fears that she and her mother, the Queen's maid, may get the blame and asks the mice for help. The trail leads to the Tower, where the Tower Ghost is haunting. However, this turns out to be the tower magpie Maggie. Together they get the diamond back to Buckingham Palace. While the queen, who had not noticed any of the excitement, waves in the gallery, Alexander holds the diamond with his hands.