McGillop was a Scientist working for UNIT who was present at the National Gallery when the Zygons gained access to the Black Archive.
McGillop was at the National Gallery along with Kate Stewart and Osgood when the Doctor and Clara arrived to view the painting "Gallifrey falls no more." As the Doctor left the room he received a phone call from a future version of the Doctor telling him to verify the number and to move the painting to the Black Archive.
Later, while talking with Osgood he is accosted by a Zygon who takes his form. This Zygon and its comrades are stopped in the Black Archive when the Elventh, Tenth, and War Doctors enter the room through the painting Mcgillop moved there earlier.
Along with Kate Stewart, he conducts peace talks with the Zygons.