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 …