Although the basic premise remains the same,
this story underwent drastic rewrites to get it into this
series. For the original storyline it was the Cybermen, not the
Sontarans who were creating drones next to a tear in the time
vortex but they were on another planet rather than on Earth.
We would be introduced to two pupils from
Belinda's former school who would fall prey to the Cybermen as
they struggle to control their rogue time-drone. The
relationship between the Doctor and Belinda continued to be
turbulent, as she was responsible for the Doctor losing his
memories. This time they would be implanted into a Cyberman who
would express the Doctor's guilt about Ace in much the same way
as Sturg in the finished episode.
For their part, the Cybermen were already at war
with the Daleks and hoping to use the time drone to give them
When I decided to make this series a precursor
for the Dalek/Cyber war obviously I could no longer use the
Cybermen. By switching to the Sontarans instead it gave me the
impetus to put Sontarans in the year's opening story Empire
of the Time Lords.
All the TARDIS interior scenes were done 22 Jan 2012 and aside
from the Sontaran ship interior (shot at the same time as
Empire of the Time Lords on 3 March) and the roof tops
scenes (done with Paradox Lost on 3 April) filming
for this story specifically was done on 9th April 2012 starting
with the nursing home corridors, the ward and finally the
The green screen elements, including Belinda in the time vortex
and the nursing home exteriors were done on 3 May.