England, 1923. Love is in the air, and all eyes are on Windsor Castle, where the royal family is celebrating a new royal engagement. When party invitations are sent to only an elite few, Dora and Rex's invite comes with strings attached. A traitor is hiding within the prince's inner circle. Dora and Rex, England's preeminent undercover spy team, are assigned to uncover the turncoat. Their sole lead is a tip from an anonymous informant. To get the clue, they must meet him privately during the engagement bash. But when they show up at the secret rendezvous, instead of a clue, they find a dead body and Rex's best friend Clark holding the murder weapon. Has Clark turned traitor, or was he framed? Can Dora and Rex work double-time to clear their friend's name and identify the real killer before Scotland Yard's leading investigator puts Clark away for life?