The secret society of very important post

It's 1953, just days from the coronation of Queen Elizabeth, and Penny Black can't get into the summer holiday spirit. Not when she's about to move to Scotland - and away from everything she knows - for good. But when Wishyouwas plops into the fireplace in a cloud of soot and chaos, with a summons from the Royal Postmistress herself, all of Penny's plans suddenly go up in smoke. The intrepid pair are soon on the trail of a mysterious traitor and a potentially deadly plot to halt the coronation. Their investigation will take them across London's busy streets, high up in the air and deeper into danger than they've ever been before ...