Doctor Who is a show about a dotty alien who travels through time and space with a neverending series of human friends. And the most important thing about those humans might be that they always go away, in the end. Some of them die, some of them marry aliens that they just met a few hours earlier, some of them get Time Lord lobotomies.

So is Doctor Who just a series of tragic stories about humans who fall away from the Time Lord's side? And what does this dismal track record say about the Ponds' chances? Spoilers ahead...

Thus far, Amy and Rory, have kept coming back, over and over, after leaving the Doctor. They manage to have a life away from the Doctor, but also keep traveling with him. Will the Ponds be the first companions to break the cycle once and for all?

In one sense, the Ponds left the Doctor at the end of last year's "The God Complex," and everything since then has been just one long coda. In "The God Complex, the Doctor realizes that Amy's faith in him is about to destroy her — so he talks her out of believing in him quite so much. And then the Doctor summarily drops Amy and Rory off at a brand new house, telling them that he wants to move on before he has to stand over their dead bodies.

But since then, the Doctor's kept coming back for the Ponds, and they've kept going away on adventures with him. Before this season started airing, we were treated to a five-minute series of webisodes, "Pond Life," which showed just how impossible it is for the Ponds to have a normal existence when the Doctor is constantly bursting in on them in the middle of the night or accidentally leaving an Ood in their bathroom.

In some ways, the business of the Ponds having a normal life interrupted by adventures with the Doctor is a throwback to the Russell T. Davies era. Davies' big innovation for Doctor Who was the idea that the Doctor's companions would still have a life back on Earth, which we would revisit every few stories. So Rose Tyler kept reconnecting with Jackie, Martha's parents became important to the Saxon storyline, and Donna Noble had her mother, and of course Wilf.

Now that the Ponds are spending more time at home, we're getting a Wilf stand-in — Rory's father Brian, who's every bit as delightful as Wilf ever was. And in fact, "The Power of Three" was one of the most Davies-esque stories we've seen since Davies left.

"The Power of Three" had a pretty wafer-thin plot, that was in the service of a story about Amy and Rory feeling torn between their "Doctor life" and "real life." And in fact, "Pond Life" feels like it's particularly a companion piece to this story — I watched them back to back, and they segue together pretty seamlessly except for the "Ponds are getting divorced" bit in "Pond Life." (Remember the Ponds' impending divorce? Me neither.)

As a story about companions trying to figure out whether to keep traveling with the Doctor, "Power of Three" was top notch. And including Brian in the mix made it much better — notable for that scene up top where Brian asks what happened to the others who traveled with the Doctor. The Doctor admits, sheepishly, that a few companions have snuffed it over the years — but insists that won't happen to the Ponds. We've now had so many pieces of foreshadowing of the Ponds' deaths that I'm calling it — they're completely safe.

(I almost titled this recap "Doctor Who and the Curse of Adric," after the companion who died in 1982. But I figured most people don't even know who Adric was. Poor chap.)

In any case, the Ponds are realizing that the constant adventuring has kept them from putting down roots, as they've wanted to since the Doctor gave them that house. Rory is being asked to go full-time as a nurse, Amy is being asked to be a bridesmaid at someone's wedding. They need to keep their fridge stocked with non-expired milk, etc. etc. And when, due to plot shenanigans, the Doctor tries to stick around for a while in the Ponds' home, he has a hard time dealing with the dullness of ordinary life for even a day.

Later, when the Doctor swoops in for the Ponds' wedding anniversary, he takes them away for a quick honeymoon which turns into thwarting a Zygon plot and Amy getting married to Henry VIII. Because the Doctor just can't ever do anything simple. (Yay Zygons!) And that's what leads to that heart-to-hearts with Brian, where Brian is worried about what the Doctor's lifestyle will do to his son and daughter-in-law.

Later, the Doctor and Amy have their own chat, where Amy points out that the Doctor was the one who dropped them off and gave them a house, encouraging them to put down roots — and "the traveling is starting to feel like running away." The Doctor counters that he's not running away, he's running to things, because the universe is full of short-lived wonders. And he keeps revisiting the Ponds because he's running to them, too, before they fade away from him. Amy's face is engraved on his hearts, because she was the first face he saw after he regenerated — which makes you wonder if the Second Doctor was obsessed with Ben and Polly, and just did a really good job of hiding it.

Oh, and once again Amy gets to be resourceful — this time restarting the Doctor's heart after it's been zapped.

In the end, it's Brian, not Amy or Rory, who decides that they should keep traveling with the Doctor — for the eminently sensible reason that the Ponds get to go off and save whole planets, something nobody else gets to do. And they should take advantage of that opportunity while they can. The Doctor promises to bring them back safe — more foreshadowing! — and we close with a cheesy voiceover where Amy recites the episode's title.

But meanwhile, the theme of companions leaving is underlined, because we meet Kate Stewart — the daughter of the Brigadier, the aforementioned poster boy for successful post-companion life. The Doctor's influence on the Brig was entirely excellent, teaching him that "science leads" and inspiring him to help save the world over and over again. And Kate (who actually debuted in some Reeltime fan-made movies years ago) is both following in her dad's footsteps and honoring Alistair's bond with the offbeat alien who always uses science to defeat incomprehensible threats.

We don't really get that much of Kate in this episode — just enough to give a mouthpiece to the returning UNIT, and to remind us that some Doctor-companion relationships end splendidly.

Oh, so the plot — basically there are these creatures called the Shakri, who are so powerful even the Time Lords told scary stories about them. They're the "pest control of the universe," and they've decided humanity needs to be wiped out before we can colonize space. So they send zillions of identical black cubes to Earth, which humans immediately adopt and take into our homes, so that after a year they can use the cubes to study us and then find a way to kill us — by stopping our hearts with electricity. (As Brian points out early on, the cubes could have just been bombs, and we'd have been screwed.) Luckily, the Doctor only needs about 10 seconds to reprogram the cubes to restart the hearts of everyone who suffered heart failure, and somehow the lengthy period in between the hearts stopping and restarting hasn't done anybody too much harm. Yay! Oh, and they have scary weird-mouth nurses and a cute android and they kidnap people from a hospital.

(I actually wonder if the Shakri will be back later this season, given how much build-up they got and how quickly they were sent packing. And that Shakri representative was only a recording. I could see the existing "through all of time and none" being a plot point in some later story.)

All in all, the plot wasn't that great — but it so wasn't the point. The point was that this is practically our last chance to explore why Amy and Rory are still traveling with the Doctor, after they've already gotten themselves a life. And to introduce the notion that, yes, traveling with the Doctor is dangerous and companions don't typically stick with him until they die of old age — the Brigadier notwithstanding.