In Blood of the Daleks, Lucie tells the Doctor that the Time Lords have placed her in his care because she has seen something important; she doesn't know what. Her initial attitude towards the Doctor is disdainful, but she recognizes that he's "about saving worlds".
In Horror of Glam Rock, Lucie unexpectedly encounters her 'Auntie Pat', Patricia Ryder, the drummer from a band called Methylated Spirits, in 1974. As this is over a decade before Lucie is born, Pat is sceptical, but grows to like her as the story develops. We also learn that Lucie's mother is named Mary.
After several more adventures with the Doctor the events of Human Resources reveal much of the mystery surrounding Lucie - the Time Lords learnt that the Celestial Intervention Agency have been tracking a woman who would be a future dictator within Europe, and alter her past to prevent this. When Lucie applies for a job which is part of another CIA plot, the primary faction of Time Lords place her with the Doctor to protect her from the effects of multiple exposure to CIA equipment. However in reality a woman named Karen, who had been interviewed on the same day as Lucie for the job, is the actual future dictator, and the Time Lords have either been tricked by the CIA or simply made a mistake. At the end of the story Lucie and the Doctor continue to travel together in the TARDIS.