The TARDIS is hit by a powerful solar flare and lands on Earth in the 22nd Century. The Doctor (Matt Smith), Amy (Karen Gillan), and Rory (Arthur Darvill) step out onto a remote island, where a factory in an old castle monastery pumps a valuable, highly corrosive acid to the mainland. The crew uses a self-replicating fluid called the Flesh to create doppelgangers of themselves. The Gangers, as they call them, are controlled by special harnesses in the dangerous environment.

The Doctor, posing as a weatherman, is worried that worse solar flares are coming and offers to take everyone away in the TARDIS. The foreman Miranda Cleaves (Raquel Cassidy) refuses to shut down the factory without an okay from the mainland. As the solar storm begins, the Doctor tries to disconnect the solar collector, but the solar tsunami hits, knocking the Doctor off the tower and rendering everybody unconscious for an hour.

When the crew awakens, they find no sign of the Gangers. It seems the Gangers are now sentient and two of them are posing as Cleaves and Jennifer (Sarah Smart). They give themselves away by turning pale white. Jennifer can also stretch her body. Confused, she befriends Rory. The new Ganger Cleaves works in secret with the other Gangers to kill the humans, and the real Cleaves plans to kill the Gangers. The Doctor cannot unite the two sides because the human Cleaves succeeds in killing a Ganger.

The Gangers plan an attack. Cleaves does not believe that what she killed was a human being. The Ganger Jennifer hunts her human self. The Doctor directs everyone to the chapel, the safest structure during the storm. The Gangers come in with acid protection suits. Rory separates from the group to find Jennifer. In the chapel, the Doctor and Amy discover a Ganger of the Doctor. To be continued.

The episode was filmed at Caerphilly Castle in Wales. Reviews were generally positive, though some of the computer-generated effects were not convincing. The Gangers were created by a combination of body doubles, makeup, and CGI. The story was thought of as The Thing meets Name of the Rose. The Doctor had been cloned or duplicated in some way eight times previously. By the end of this episode, everyone in the TARDIS has been duplicated. In fact, the Doctor is there to study The Flesh because he thinks Amy is not real.