After last week's adventure, the Doctor and Clara have picked up two new (miniature) companions. Clara decides to take the children, Angie and Artie, to an extraterrestrial theme park.
Unfortunately, the park appears to be closed down, and even worse, under some sort of military occupation. The Doctor manages to convince the military that he's there to help them search for their missing emperor, and the group meets up with Webley, the owner of the theme park.
At this point in time, the Cybermen have been defeated after a brutal war with humanity. Now it seems a few shells exist as harmless museum exhibits, one of which Webley has converted into a futuristic Turk (piloted by his friend Porridge).
Unfortunately, it seems the Cybermen were not quite as extinct as they seemed, and a group of mysterious robot bugs reactivate the museum Cybermen. The metal men soon abduct Webley, Angie, and Artie, converting them to cyborgs.
Meanwhile, it turns out that the military unit is actually a bunch of rejects sent on this mission as a punishment (and to keep them out of the way of the real investigation). Even worse, humanity's method of defeating the Cybermen the first time around is revealed: whenever Cybermen are found on a planet, that planet is immediately imploded. Apparently, this was the only way to stem the invasion, as the Cybermen forces only grow stronger after each battle. It's now a race against time to defeat the metal monsters before the military is forced to destroy the entire planet with a bomb.
Things get even worse when the Doctor himself is captured and given cybernetic parts. An entity known as the Cyber-Planner infects the Doctor's mind, and attempts to gain control of his brain. The Doctor fights back, and realizing they are at a stalemate, the two agree to play a game of chess to decide who gets control. At the same time, it turns out Porridge was the emperor all along, who decided he didn't want to be in charge anymore.
A back and forth battle of wits between the Doctor and the Cyber-Planner occurs, with Clara leading the troops in a battle against the growing number of Cybermen. In the nick of time, the Doctor manages to out bluff his chess opponent and banish the Cyber-Planner from his brain.
Porridge then rescues everyone by activating the bomb himself, which summons his flagship to teleport them all away as the planet is destroyed. Porridge scans the planet, which reveals no surviving Cybermen.
All seems well, until the closing shot reveals one Cyber bug has survived... Perhaps a hint of a future episode?