premise: He often referred to the royal garden party when he was in a good frame of mind. Scarlet still found the episode entirely mysterious, since she could think of no reason at all why they had been invited. It was improbable in the extreme that Brian's sterling qualities had come to the notice of the Comptroller of the Household and she could only assume that the summons had been the consequence of her father's having been an RA.
hypothesis: the summons had been the consequence of Scarlet's father's having been an RA