Amy (Karen Gillan) and Rory (Arthur Darvill) are adjusting to normal life without the Doctor (Matt Smith), until billions of small black cubes appear on Earth. The Doctor arrives and he and his two companions are taken to UNIT Headquarters, where Kate Stewart (Jemma Redgrave), daughter of the late Brigadier Lethbridge-Stewart, explains that they are at a loss about the function of the cubes. The Doctor waits for data with Amy and Rory and gets to know Rory’s father Brian (Mark Williams) a little better. The Doctor concludes that the cubes are harmless and leaves.

A year later, after Amy and Rory’s latest trip with the Doctor, Brian asks the Doctor what happened to his past companions. Some, the Doctor admits, met a sad fate, but he will not let that happen to Rory and Amy. Humanity has stopped worrying about the cubes and uses them for paperweights.

On the one-year anniversary of their arrival, the cubes activate, scanning information and protecting themselves. The Doctor calls this a slow invasion and reports to UNIT with Amy, while Rory and Brian help out at the hospital. The cubes are counting down. At zero, they open and are empty, but massive numbers of heart-attacks kill a third of humanity. Seven sites on Earth send signals, including the hospital where Rory works.

Rory discovers a portal behind a service lift leads to a ship in orbit. Brian is caught up in it, but the others free him. The Doctor finds a hologram of a Shakri (Steven Berkhoff) which in Gallileyan legend are the pest-controllers of the Universe. The cubes are there to wipe out humanity, but the Doctor reverses their function, bringing everyone back to life. The four escape just before the Shakri ship explodes. The Doctor leaves, taking Amy and Rory for one last trip.

The story was basically about Amy and Rory, as the next episode would be their last. It was well reviewed, though considered a bit rushed at the end. The Tower of London is UNIT Headquarters. Steven Berkhoff only had eight lines as the villain because he purposely ruined several takes. This was in fact the last episode filmed with Karen Gillan and Arthur Darvill. The opening logo was unique in several ways.