Inspirations: The Power of Three

It was originally intended that the first half of Series 7 would only comprise four episodes, and this story was actually a late addition. Chris Chibnall was given the job as the production team were happy with his dinosaur script. He hadn't written a big invasion story, and Moffat had asked for a storyline that spanned a whole year.

As a starting point, the writer recalled a story about a container ship caught in a storm off the coast of Devon. Some of the containers were washed overboard and came to shore - and the local populace had a field day raiding them for their contents. This led to the idea of people being presented with something for free and how they would react to it. The stages to go through would be initial excitement, then growing complacency, before the objects suddenly turned on them.

An Earth invasion prompted thoughts of bringing back UNIT, Chibnall being a big fan of the Pertwee era as could be seen in his Series 5 Silurian story.

With Nicholas Courtney now passed, it was felt to be a good idea to keep the family connection with the Brigadier. Spin-off media, such as the video production Downtime, had given him a daughter named Kate.

Novels had her the daughter from his first marriage, whilst Chibnall himself thought of her as the child of the Brigadier and Doris.

Chibnall was also able to bring back Brian Williams, Rory's dad, having introduced him in his earlier story.

The Doctor at one point speaks to him about keeping his companions safe, admitting that it has not always been possible - referencing Katarina, Sara Kingdom and Adric.

For a time, this story was going to be called "Cubed", and mathematically a number (N) cubed means that it is multiplied to the power of three - i.e. N x N x N.

The Power of Three therefore alludes both to the mysterious cubes which appear on Earth, and the idea that the Doctor at this particular time generally triumphs when backed by his two companions.

Prior to the beginning of Series 7, Chibnall had given us the mini-series Pond Life. This comprised short episodes illustrating what the Doctor was getting up to, now that Amy and Rory no longer travelled full-time in the TARDIS, and - more importantly - how they lived their life day-to-day in settled domesticity in his absence.

Steven Moffat described it in terms of the Doctor's impact on the couple as being akin to the movie The Man Who Came To Dinner (1942).

The Power of Three feels like a continuation of minisodes. There is the over-arching mystery of the black cubes, but for the most part it has an episodic feel. Like Pond Life, we dip into the lives of the couple over the twelve months, seeing brief adventures such as Zygons in late Victorian London, as we go.

Next time: Angels in America...