The secret society of very important post

It's 1953, just days from the coronation of Queen Elizabeth II, and Penny Black can't wait to go on her summer holiday to Scotland to visit her Uncle Frank. But when Wishyouwas plops into the fireplace in a cloud of soot and chaos, with a summons from the Royal Postmistress herself, all 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.