The Royal Yot

Aware that a hostile empire has designs on several of its members, the Sovereign Planet Alliance (SPA) is keen to add the newly discovered planet of Xinthuva to its ranks and gain access to top-notch Xinthuvan defence technology. Trouble is, the Xinthuvans are staunch royalists. So staunch, that membership negotiations cannot even begin until they've held a magnificent royal get-together for their royal head of state and the royal heads of state of each and every Alliance planet.
Unfortunately, a royal head of state is something most SPA worlds no longer have. Even after they all manage to come up with suitably regal, albeit, in some cases, reluctant, representatives for this vital gathering, worrisome incidents on the specially commissioned starliner making its way to Xinthuva lead its captain to suspect that someone aboard his ship does not want this gathering to be a success.
But who among the royal envoys is targeting the others? And why, when bringing Xinthuva into the Alliance stands to benefit them all?