Dr Larsen was a microbiologist in the employ of Heights Pharmaceuticals. She worked under Dr Frances Godalming at the Emergency Response Centre during the Invictus outbreak.
After taking blood from Jack Harkness and working towards a vaccine, Dr Larsen confronted Dr Godalming about her announcement of Provictus as a cure. When she admitted that the Provictus strain was active, she joined Jack in stopping it, getting him access to the Data Core and later managing to create a genuine vaccine. She also tended to Rhys Williams's gunshot wound. (AUDIO: "Invasion: Kill")