Petronella Osgood was an enthusiastic young scientist who worked for UNIT. Her dedication to her work was second only to her devotion to the Doctor. Her attire often reflected her admiration of the Time Lord, sometimes wearing a long scarf or a bowtie or even question-mark lapels to reflect the wardrobe of various regenerations of the Doctor.
She finally got to meet the Doctor when Kate Stewart brought him to the National Gallery to investigate several paintings that had been smashed from the inside. Keen to impress the Doctor, Osgood proved herself to be resourceful, tripping up a Zygon with her scarf to escape from its clutches. Osgood and a Zygon duplicate helped broker peace talks between the species.
Later, Missy murdered one of these Osgoods, but the surviving one refused to reveal whether they were Zygon or human. Briefly, this Osgood became target of a rogue Zygon group who believed she had betrayed their species.
In the end, the leader of the group not only abandoned their antagonism, she transformed her appearance into that of Osgood. Both of these Osgoods then dedicated themselves to defending the Earth.