Evaline Stoker and Mina Holmes have reluctantly agreed to act as social chaperones and undercover bodyguards for Princess Lurelia of Betrovia, who has arrived in London to deliver a letter that details the secret location of an ancient chess queen that's been missing for centuries. But when the letter-which will heal a centuries-old rift between England and the Betrovians-is stolen out from under Evaline and Mina's watchful eyes, the two girls are forced into a high-stakes race to ensure they find the chess queen before anyone else does.