After three fairly straight forward adventures, "The Power of Three" introduces a welcome element of mystery into the current season of Doctor Who, and manages to tie its central conundrum into the changing relationship between Rory, Amy and the Doctor for good measure.
The premise is a cracker: overnight, billions of small black cubes appear all over the Earth. Nobody knows how they arrived. Nobody knows what they do. It's an enjoyably low key concept but the Doctor doesn't do low key though, and as the cubes continue to do nothing, his impatience gets the better of him. Rory and Amy, settled into their normal lives and contemplating an end to their travels through time and space, are quite happy to wait and see what the cubes do.
Days pass. Then weeks. Then months. In a show that often favors an urgent mood, the “year of the slow invasion” is a fantastic way to break out of formula. It's also the sort of idea that harks back to the sort of post-war British sci-fi that inspired Doctor Who in the first place. This is the sort of weird yet mundane turn of events that could feature in a John Wyndham novel or a Quatermass teleplay.
The script has a lot of fun with both the curious nature of the plot and the languid pace at which it unfolds. There are cameos from astrophysicist Brian Cox and UK Apprentice star Sir Alan Sugar, as well as various real British newsreaders, all helping to sell the notion that these cubes have become taken for granted. The revived Who hasn't had a great track record in using such pop culture nods – witness the terrible Anne Robinson robot and Weakest Link parody in the Christopher Eccleston series finale, "Bad Wolf" – but these work very well, bridging the gap between the real world and the fiction of the Doctor in a way that echoes Rory and Amy's dilemma, rather than simply winking at the audience.
We also get to see the Doctor playing Wii Sports, and whisking Rory and Amy off for an anniversary treat – a romantic night at the Savoy hotel in 1890, which we then learn was disrupted by a Zygon spaceship hidden beneath the building, which then somehow leads to an encounter with Henry VIII (that'll be where Rory left his phone charger in the king's commode, as mentioned last week then). It's playful stuff, and the spritely tone is matched with some lovely callbacks to both new and classic Who. We see Amy and Rory sharing a bowl of fish fingers and custard with the Doctor, and there's also an affectionate nod to K-9, the Doctor's “metal dog."
Not least, the introduction of Jemma Redgrave as Kate Stewart and the reintroduction of UNIT. This was the quasi-military intelligence faction which kept the third Doctor, Jon Pertwee, busy for much of the show's seventh season back in 1970. UNIT was reintroduced during David Tennant's run, but never really woven back into the continuity. Having UNIT leading the investigation into the cubes, rather than the supposedly edgier and sexier Torchwood, bodes well for future stories using this popular element of Who lore.
Having Redgrave play the daughter of the Brigadier, one of the best loved supporting characters from the show's past, is another delightful touch – especially as this story could easily have fitted into the Pertwee era, when the Doctor was exiled on Earth and forced to pass the time by helping UNIT deal with extra-terrestrial threats.
And, of course, the cubes do indeed turn out to be an extra-terrestrial threat. They've been playing the long game, waiting for humanity to stop paying attention to them, gathering information on our biology and culture, before activating and zapping the nearest person with a heart-stopping jolt of electricity. There's a creepy human child in the hospital where Rory works. Two porters with weird spout-like mouths subdue and kidnap an old man. Later, they do the same to Rory's dad. Rory follows them into an elevator which doubles as a portal to what the Doctor explains as “the next dimension to the left” where the cubes are apparently controlled by a spaceship.
It's here that the episode wobbles, as what started as a thoroughly intriguing mystery gets resolved in a disappointingly rushed and not entirely logical way. The Doctor identifies the threat as the Shakri, a wandering race of galactic “pest controllers” that were considered a myth on the Doctor's home planet of Gallifrey. The Shakri are represented by Steven Berkoff, buried under make-up that looks halfway between a Borg and a Cenobite, as he intones that the plague of humanity must be wiped out before it can spread to the rest of universe.
None of which makes much sense. For one, the cubes seem like a horribly inefficient way of eradicating a species, especially as the Shakri seem to know everything about us, making the yearlong infiltration by the cubes strangely pointless. More pressing is the question of just what humanity is supposed to have done to deserve genocide. This is a universe populated by Daleks, Cybermen, Sontarans and countless other warlike species, hellbent on conquest and enslavement, yet the Shakri target Earth? That's some weird priorities.
Not that any of this really matters, as the threat is resolved with unusual haste. The Doctor simply waves his sonic screwdriver at a screen and the cubes instantly go from killing machines to tiny defibrillators, bringing their victims back to life. As resolutions go, it's incredibly slim and anti-climactic. As for the Shakri, Berkoff is a hologram, he disappears and the fact that there's still a sinister mythical alien race looking to kill every human on the planet is brushed aside. The young girl in the hospital is revealed to be some sort of robot, for no apparent reason, and the spout-faced porters and their kidnap victims are never really explained either. There are enough loose plot threads to knit a whole new scarf for Tom Baker.
But none of that really matters, because the threat in this episode is ultimately just a MacGuffin to explore the relationship between the Doctor, Amy and Rory. There's a wonderful scene between the Doctor and Amy, where she confesses that she feels like her and Rory's adventures in the TARDIS are becoming an escape, enabling them to run away from starting their married life together. In reply, the Doctor offers a rather beautiful explanation for his seemingly aimless wandering in a universe that is constantly changing, destroying and creating. “I'm not running away from things,” he insists. “I'm running towards them, before they flare and fade."
The Doctor seems to have made his peace with the idea of Rory and Amy fading from his life, but the end of this story finds Rory's dad (another superb turn from Mark Williams) urging them to continue travelling through time and space. His argument is simple but compelling – who else gets to do this? - but enough hints about companion mortality have been dropped to suggest that their ending may not be a happy one. One way or another, the time to say goodbye for good is rapidly approaching.