First broadcast 22 September 2012.

One day, millions of small black cubes appear on the planet Earth. No one knows where they came from, and no one knows what they are for. Even the Doctor (Matt Smith) cannot work out their purpose, leading to a year-long search for answers – and a year of living with the Ponds.

“The Power of Three” is a highly idiosyncratic episode of Doctor Who. It stands out as a genuine attempt to try something new for the series. It kicks off a new era for paramilitary force UNIT with its introduction of recurring character Kate Stewart (Jemma Redgrave). At the same time it struggles enormously due to behind-the-scenes troubles during its shoot. Those troubles do compromise the final result terribly, but in the main this is still a memorable and very enjoyable episode. I remember when first watching it thinking that writer Chris Chibnall – who also wrote this season’s “Dinosaurs on a Spaceship” – might actually make a decent successor to Stephen Moffat as series showrunner. Of course that is precisely what happened, and I think one can see a lot of Chibnall’s 2018-22 run as producer in a nascent form here.

There’s a lovely languid pace to “The Power of Three”. It honestly lets the Doctor relax into everyday life. There is the mysterious cube invasion going on, but honestly the episode is actually about Rory (Arthur Darvill) and Amy (Karen Gillan) making a choice: should they continue travelling with the Doctor or actually break away and establish their own lives on Earth? It is unusual for Doctor Who to foreshadow the departure of companions like this, with a strong sense of dramatic irony and proper character development. There are some beautiful scenes through this episode: the Doctor and Amy sitting watching the Thames outside the Tower of London, and the Doctor admitting to Rory’s dad Brian (the returning Mark Williams) that not all of his companions survive. It has been lovely having Williams in the series during Season 7, and he’s been utilised very well both times he has appeared.

The introduction of Kate Stewart finally gives UNIT an actual purpose and a strong appeal. While the agency has appeared in the 21st century series before, it has always been as a somewhat anonymous group. Introducing a memorable character at its head at last brings it more in line with its original early 1970s run, and making Kate the daughter of the late Brigadier Lethbridge-Stewart is a lovely touch – even if the idea of replacing soldiers with scientists does get rather side-lined over the following decade.

The actual invasion plot is rather lacklustre: there’s only so much you can do with inert black cubes. The episode’s climax is an absolute mess. At the time it was only possible to blame Chibnall’s writing or Douglas Mackinnon’s direction. It is only in retrospect that fans learned the real problem was with guest star Steven Berkoff, who point-blank refused to recite his scripted dialogue. With a tight production schedule, there was no opportunity to reshoot or recast. Mackinnon was forced to cobble together what he could during post-production using whatever random things Berkoff chose to say as well. Given the circumstances the climax is surprisingly effective – even if it fails to make a lick of sense.