Mairi Ellison was a doctor who worked at the Parkland Memorial Hospital. On 23 November 1963, Mairi encountered the Eleventh Doctor and Clara Oswald examining a patient, Mae Callon, in her office. After she asked them what they were doing, the Doctor showed her the Shroud that had manifested as Mae's grandmother, Betty Callon, on Mae's arm, and Mairi backed away in fear of it. In the midst of this, two screams were heard the hospital's hallways, and Mairi and Clara went to investigate one of them.
As they arrived to the source, they came across Andrew Ross, who worked as a pupil under Mairi. As they walked up to him, they saw a face in a spilled blood bag, and Mairi explained that the face was Andrew's wife, Sophie, who had died in a car accident with another man, Chet. Even as Mairi tearfully tried to calm him down, Andrew eventually took his life due to the grief of the Shroud. Later, Clara said that Mairi had "gone to be with a late friend".
While the Doctor was returning to his vehicle, he noticed that Mairi had succumbed to the Shroud herself, the creature having taken the form of an older man (presumed to be her father by Clara). He promised to her that he would free her from the Shroud's grasp, but for the moment, he couldn't do anything. Mairi ended up being one of the people that the Doctor would later "heal" with the Once More With Feeling, and she gave him her thanks.
After the group had successfully defeated the Shroud, Mairi, along with Captain Adam Keating, led General Harley B. West to believe that he had combated the Shroud using a chemical attack, but that it had resulted in memory loss, with Mairi serving as his medical consultant. They convinced him to retire after "the highlight of [his] career", and subsequently gave him a coffee laced with a sedative. (PROSE: Shroud of Sorrow)