The Doctor has popped in and out of the lives of Amy Pond and Rory Williams for the last three seasons (or in their time, ten years). Just as they’re getting used to life at home, the Doctor pops back in again in the episode “The Power of Three,” as do millions of black cubes worldwide. The enigmatic boxes do absolutely nothing but sit.
The Doctor is understandably curious and concerned. So is UNIT, the military branch of the United Nations that deals with extraterrestrial threats. (They played a huge part in Doctor Who during Jon Pertwee’s tenure, from 1970-1974.) UNIT’s scientific advisor, Kate Stewart, tells the Doctor that rounding them all up would be impossible. So the Doctor advises that everyone observe them.
Brian (Rory’s dad, from “Dinosaurs on a Spaceship,”) takes this advice to heart and monitors them carefully. As for the Doctor, ennui gets the better of him. Four days of sitting around (and painting fences and playing with a soccer ball) get the better of him, and he runs off on another adventure.
Amy and Rory settle back into everyday, non-Doctor life, and for the first time in a decade make plans in advance. “Did real life just get started?” Rory asks. “I like it,” Amy says.
Meanwhile, Rory, a hospital nurse, goes to work, oblivious to a young girl with a cube whose brown eyes flash blue. Nearby, a patient impatiently waits for a prescription. One, and then two Evil Medics attack him and take him away. (We know they’re evil because their mouths are creep-tastic.)
Months later, the Doctor reappears to take Amy and Rory away as an anniversary gift, which turns into a seven-week ordeal. When they return, Brian buttonholes the Doctor and asks him about the consequences of their trips. The Doctor, carefully and painfully, admits, “Some left me, some got left behind, and some, not many, but some died. Not them. Not them, Brian. Never them.”
It turns out the Doctor wants to stay, not only to keep an eye out on the cubes but also—possibly a first-time admission for the Doctor—because he misses Amy and Rory. They settle into domestic life, sharing fish fingers and custard at night. Shortly after, Brian, Amy, Rory, and the Doctor simultaneously notice activity from different cubes, with the Doctor fleeing from a cube that somehow wields laser beams.
Rory and Brian go to the hospital where Rory works to help out with the inevitable medical emergencies, while the Doctor is summoned by UNIT, and brings Amy as a backup. All of the cubes, it turns out, have reacted differently. (It also turns out that Kate Stewart is the daughter of Brigadier Alistair Lethbridge-Stewart. For those of you who don’t know, he’s a beloved character, played by the late Nicholas Courtney on and off for years.)
Amy and the Doctor have a heart to heart about her and Rory stopping traveling with the Doctor, when the Doctor realizes that the cubes have scanned the entire planet, to acquire information. And suddenly, the cubes start glowing with the number seven, which soon counts down to zero. Then the cubes open. Nothing. At first.
Worldwide, many people fall down, victims of heart attacks caused by surges in electrical energy pointed at human hearts. One of the Doctor’s two hearts shuts down.
Meanwhile, Brian is captured by the Evil Medics, and Rory chases after them…all the way to an alien spaceship orbiting Earth, via a portal hidden inside a freight elevator.
Amy and the Doctor go to Rory’s hospital, and when the Doctor’s second heart gives, Amy revives him. The creepy girl at the hospital, it turns out, is a droid monitoring the planet. The Doctor dispatches her with a wave of his sonic screwdriver.