10 Doctor Who Plot Holes You Didn't Realise Were Actually Solved
6. How Doesn't The Doctor Know Which Osgood Survived?
Ever since their first appearance in 1975, the Zygons have had clearly established powers. They can shapeshift into the form of a humanoid creature with near-perfect accuracy, but they need to have a living host to copy. If the host either dies or is rescued, the Zygon cannot retain its shape and will revert back to a “big red rubbery thing covered in suckers,” as the Tenth Doctor so eloquently put it.
This has left some fans slightly confused about the Doctor’s confusion as to which version of Osgood was killed by Missy in Death in Heaven, and which one survived. After all, if the human Osgood had died, the Zygon version wouldn’t be able to remain looking like Osgood. So it must have been the Zygon that was killed.
But as we discover in Series 9’s Zygon two-parter, things are no longer that simple, with Osgood describing the commonly understood system as “the old rules.” It turns out at some point before these episodes, the Zygons underwent a slight upgrade, and are now capable of shapeshifting into the form of anybody they’ve already scanned. Osgood says:
"Those were the old rules, before Zygons could pluck loved ones from your memory and wear their faces. Zygons only need to keep the original alive if they need more information from them. If the interrogation is over, then the original can die."
As if the Zygons weren’t terrifying enough already...