The Doctor (Matt Smith) takes Clara (Jenna-Louise Coleman) and the Maitland children she is babysitting, Angie (Eve De Leon Allen) and Artie (Kassius Carey Johnson), to an extra-terrestrial theme park. They find that the park is closed and occupied by a punishment platoon. The Doctor poses as an ambassador working for the missing emperor and they are allowed to enter.
There are deactivated Cybermen in the park, considered harmless because their kind was defeated a thousand years ago. Naturally, an army of functioning Cybermen is hiding under the park and upgrading themselves over time. They wake up and steal Angie and Artie. The Doctor puts Clara in charge of the platoon, warning her not to destroy the planet while he rescues the children.
He finds Angie and Artie, but is partially upgraded into a Cyberman, which means he is able to share their super-consciousness, called the Cyberiad. This gives him a split personality, the Doctor and the Cyber-planner, whom he calls Mr Clever. They play chess for control of his body. The Doctor puts a golden ticket on Mr Clever’s face because Cybermen are allergic to gold, but Mr Clever installs a patch to bypass this.
Clair relocates the platoon to a comical castle in the theme park where they arm themselves with a large anti-Cyberman gun, five hand pulsers, and a planet-imploding bomb. The Doctor returns with Angie and Artie, and asks to be tied up to finish his chess game, while the troops do their best to hold off the Cybermen, who are constantly upgrading themselves like the Borg.
Mr Clever destroys the bomb trigger, the Doctor uses a hand-pulser to remove Dr Clever from his mind, and Angie recognizes that a man named Porridge (Warwick Davis), from a picture on an Imperial penny, is the missing Emperor. The Emperor activates the bomb and signals an Imperial spaceship to teleport them away. Just before the bomb implodes, he retrieves the TARDIS. One Cyberman survives, floating in space.
The story was written by Neil Gaiman, who had been asked to make the Cybermen scary again. As he was giving his Hugo Award acceptance speech for the Doctor’s Wife episode, he said only a fool would try to write for Doctor Who again, so he was on his third draft. He decided to amalgamate the early Cybermen, who were alien cyborgs from the planet Mondas, with the later Cybermen, who were human cyborgs from another dimension. Warwick Davis was thrilled to be in a Doctor Who written by Neil Gaiman. One of the children in the cast lost the script in a taxicab and it was soon all over the Internet. The episode was enjoyed by the public and well received by critics.
The similarity of these Cybermen to the Borg was noticed, but of course the Borg had been inspired by the Cybermen in the first place. The Cybermites were like silverfish. Gaiman was inspired by the Turk, a man-shaped, chess-playing automaton built in 1770 and destroyed by fire in 1854. It was a hoax, and a human chess master was inside. Jenna Coleman had a small role in Captain America: The First Avenger. The Cyber-Doctor reveals images of all the previous Doctors, which were used later in the Day of the Doctor. The game of Chess was invented by Time-Lords, apparently. Special effects in the episode were influenced by the Matrix movies.