The Love Invasion
Igrix was a Kustollon from Kustollia.
In 1966, Charlotte’s husband Peter mysteriously died in his laboratory after he believed Lend-a-Hand girls were following him. Charlotte investigated the Lend-a-Hand agency after scientists working on other unrelated projects had died or gone missing, and saw the Kustollon Igrix was scanning people’s minds. She killed one of the Lend-a-Hand girls, who were biotechnology artificially grown from Kustollon genestuff. By experimenting on the Lend-a-Hand girl to find her weaknesses, Charlotte developed a chemical that could dissolve the Lend-a-Hand girls’ alien biology, and began to kill them as a way of fighting back. She then helped the Ninth Doctor and Rose Tyler find out the truth about the Lend-a-Hand organisation.
Charlotte’s research into airborne sprays, combined with her husband’s work on DNA resequencing, allowed the Doctor to create a virus which made Kustollon biotechnology human, stopping Igrix from changing history. This combined research was later used to allow the humans to engineer another virus that disabled the Kustollons’ biotechnology during a battle in 3046. (The Love Invasion)