Brenna was one of the very first Ashtallan scientists. They worked alongside Sharlan. The two scientists met Barbara Wright and Susan Foreman, who introduced them to concepts which they both found fascinating. Brenna was present when Barbara was healed from a mortal wound with Ashtallan cells. While they didn't understand the concept, Brenna loved Sharlan in much the same way that Sharlan loved them. When Ashtallans started dying, they assisted the First Doctor in finding the cause.
The Doctor succeeded in creating a child using cells from both Brenna and Sharlan. When the child came at risk of being infected by the plague, Brenna managed to protect their child. They mourned Sharlan's death, in spite of the fact that they had caused the plague. (AUDIO: The Invention of Death)