The Doctor and Clara take Angie and Artie (Clara’s charges) to an Earth Empire planet in the far future to visit the greatest theme park in the galaxy, Hedgewick’s World of Wonders. Finding it laid waste after the last Cyber-War, they enounter a showman called Webley (Jason Watkins) and his assistant Porridge (Warwick Davis), and are alarmed to discover their prize exhibit is the shell of a Cyberman that has been rigged up to play chess.
As Webley boasts he owns three Cybermen, tiny glowing insect-like creatures crawl up a wall. Noticing them, the Doctor decides to investigate. The tiny cybermites activates a Cyberman that upgrades Webley and kidnaps Artie and Angie. Fearing the local military force will destroy the planet rather than save the children, the Doctor makes Clara the platoon’s commanding officer – much to the chagrin of its captain (Tamzin Outhwaite). Clara then instructs the platoon to head to Natty Longshoe’s Comical Castle, while the Doctor uses a transmat to locate the children being held by the Cyber-Planner.
The Cyber-Planner reveals to the Time Lord that the amusement park planet houses three million Cybermen, who were repaired following the Cyber-Wars using human visitors as spare parts. Now they need the brains of children to build a new cyber planet. But when the cybermites scan the Doctor’s mind, they discover the Time Lord would make a better candidate. The Cyber-Planner tries to control of the Doctor’s mind, but reaches a stalemate. The Doctor then makes the offer of a game of chess. If he wins, the children go free. If not, he will let them die.
At the castle, the Captain tries to trigger an implosion bomb, but is blasted by a Cyberman. The Doctor arrives, having blocked the mind control with a gold ticket, and reveals his plan to save Angie and Artie. But the Cyber-Planner gains control again and destroys the bomb trigger. Awoken from their tomb, the Cybermen advance on the castle, but when the Cyber-Planner struggles with the Doctor’s superior chess moves, the Cyber-Planner uses the entire compliment of Cybermen to make his next move. In doing so, the legion stops in their tracks. The Doctor then uses an amplified hand pulse weapon to shut down the Cyber-Planner and rescue the children. When Angie deduces that Porridge is, in fact, the Emperor in hiding, the Doctor and Clara convince him to beam everybody aboard an Imperial flagship orbiting the planet and detonate the bomb.
The episode ends with the Doctor delivering Clara, Angie and Artie home safely, while Porridge sets off home, unaware that a cybermite is floating in space…