The Doppleganger duplicates of workers at an island acid factory have awakened and turned against their human originals. The same solar storm that created them has also created a Ganger Doctor (Matt Smith). Rory (Arthur Darvill) has gone off to check up on the emotionally distraught Jennifer (Sarah Smart). The Ganger Jennifer kills the real Jennifer and then stages a fight with another Ganger to seem more like the real Jennifer. Then she leads Rory to a console, telling him it will restore power when it actually disables the cooling system, so the acid which the factory produces will become unstable. Finally, she convinces Rory to lead the humans into the acid storage chamber. Crewman Jimmy (Mark Bonnar) is killed in the process.

The Doctor tricks the Ganger Jimmy and the other Gangers into believing they are just as real as their human counterparts. Ganger Jennifer stalks off in a rage, while the other Gangers agree to work with the humans to escape. They free the humans trapped in the acid storage room and flee through the monastery below, chased by Ganger Jennifer, now grown monstrous. Ganger Cleaves (Raquel Cassidy) and Ganger Doctor hold the door to allow the Doctor, Amy (Karen Gillan), Rory, Human Cleaves and Gangers Jimmy and Dicken (Leon Vickers) to escape in the TARDIS. The Cleaves and Doctor Gangers face the monster, using a sonic screwdriver to cause the monster, and themselves, to dissolve back into liquid.

Aboard the TARDIS, the Doctor stabilizes the Gangers, so they become human permanently. As the TARDIS crew prepare to dematerialize, Amy has contractions. The Doctor reveals that Amy has been a Ganger for some time, and he had been interested in the Flesh because it created her. He promises to find the real Amy and dissolves her Ganger. The real Amy wakes up somewhere, pregnant in a pristine white tube, watched by the lady with the eye-patch (Francis Barber) that she has been seeing in her mind’s eye for many episodes.

The Ganger Doctor speaks several lines originally spoken by the First, Third, Fourth, and Tenth Doctors. He asks to be referred to as John Smith. Writing dialogue for two Doctors was fairly easy, since he talks to himself anyway. Reviews were favorable, though some thought the surprise cliff-hanger somewhat overshadowed the main story. Matt Smith was praised for his dual role, being reassuring, threatening, hilarious, and sinister by turns. It is because the original Jennifer had a perfect memory that her doppleganger remembered every terrible thing that was done to the Gangers.

One of the themes here is that the Gangers are just as real as their original models, but it seems a little easy for the plot to throw them away. However, there is a great deal of interesting complex interactions between originals and Gangers throughout. Ganger Jennifer reacts with so much anger that she literally evolves into a monster. Ganger Cleaves is just as noble as the original Cleaves. The Two Doctors interact like actual twins, making us think that the Doctor was kind of schizoid to start with. And when Ganger Amy realizes she is not real and is about to dissolve into nothing, it seems quite disturbing.