The Doctor and Ruby attend a party at the estate of the Duchess of Pemberton in 1813. Amongst the merry dancing and whispered scandals, they soon discover that they aren't the only interlopers. The Doctor is drawn to a bounty hunter calling himself Rogue, who's hunting a shapeshifting Chuldur. Unfortunately, he believes the Doctor himself is the Chuldur -- and he's determined to inflict a death sentence. Meanwhile, Ruby's investigation brings her perilously close to the real Chuldur: not a single alien, but an entire family lurking amongst the guests.