Death Undercover

They were sent to unmask the spy in their midst. Instead, they'll have to identify his murderer.
England, 1922. After their adventures at Westminster, Dora and Rex depart for a quiet month in the Cotswolds. Or at least, that's what everyone thinks.
In truth, they're working on a new assignment - investigating rumours that a titled foreigner might be more than he appears. Rex invites the man to his family manor for a rollicking weekend of fun. In between bouts of lawn tennis and parlour games, he and Dora are to uncover the truth about the man's identity.
The servants barely have time to unpack everyone's case before the suspected spy is found with an antique dagger sticking from his back. With so few suspects, the case shouldn't be too hard to solve. But when every door leads to a dead end, Dora and Rex are forced to consider a most unpalatable option.
Were they wrong in thinking the man was a foreign spy? Or... could someone else have a powerful motive for murder?