A nexus in the primeval cauldron of space-time itself unexpectedly deposits the TARDIS in the funfair at Blackpool, where the Doctor decides that Peri is going to have fun if it kills her. However, just as they're starting to enjoy themselves, the Doctor hears a voice in his head calling his name. He follows the voice to the amusement arcade, where he and Peri see a desperate Scottish woman searching for her missing son Tyrone. Tyrone soon reappears, with no memory of having been abducted and given extensive medical tests elsewhere in the funfair; nevertheless, his temporary disappearance has attracted the attention of a young man named Kevin Stoney, whose brother recently disappeared from this very arcade. Kevin was recently released with a warning after breaking into the funfair after hours to search for his brother, and his reports of seeing strange lights, glowing faceless giants and a Chinese mandarin have been dismissed by the police. When the Doctor hears the disembodied voice again and rushes out of the arcade in response, Kevin notices his odd behaviour and decides to follow him.
The voice lures the Doctor and Peri to the Space Mountain ride, where they are separated and the Doctor is diverted into a service tunnel. Peri attempts to complain to the ride boss, but gets nowhere until Kevin joins in and threatens to call in the police. The ride boss directs them towards the security office -- where they are taken prisoner by boiler-suited thugs. They manage to escape and flee into the shut-down Broken Neck Gap ride, but as they search for a way out, the animatronic miners come to life and begin to follow them. The miners eventually attack, knocking Kevin out and forcing Peri to flee as the guards arrive to recapture her. Kevin manages to find her again, claiming that he played dead until the guards left him, and that while searching for her he's found a way out. Eventually Peri realizes that he's leading her around in circles and that they're being followed, but when she confronts him, realizing that he isn't Kevin at all, he vanishes into thin air and she is captured by the guards.
The Doctor, meanwhile, is imprisoned in a cell which has been specially prepared for him. The pipes in his cell begin rattling rhythmically and he realizes that someone on the other end is attempting to communicate, but before he can establish a meaningful dialogue he is confronted by his captor -- the Celestial Toymaker. With a gesture, the Toymaker causes the wall separating the cells to vanish, and the Doctor is attacked by a monstrous crustacean; just in time, however, he realizes that the rattling pipe leads through both cells, and when he taps the pipe again he is able to convince the crustacean that he is an intelligent life form, the same one which was trying to communicate earlier. Pleased, the Toymaker places the Doctor in a cell with Kevin and a video game on which he is to practise his skills; when Peri is recaptured, she is placed in the cell as well. Instead of practising, however, the Doctor rigs the security cameras to broadcast a false image of the cell and takes the video game apart for its circuitry. While building a new machine out of the circuitry of the game, the Doctor explains to Peri and Kevin that the Toymaker is an immortal who delights in playing games for the ultimate stakes. His origins are unknown, as are the extent of his powers, which seem to bear no relationship to the natural laws of the Universe...
The Toymaker genially observes his technicians' progress as they struggle to overcome the last obstacles in the creation of his Great Work. The technician who solves the final problem, Yatsumoto, is rewarded by being given the first chance to play the end result, the ultimate video game. The thunderously realistic game depicts a lone player's battle against alien creatures in a blasted urban landscape, and Yatsumoto quickly loses his three video game lives -- and then his own, when one of the creatures steps out of the screen and kills him. Satisfied, and knowing that the blueprints for his game will soon be distributed to factories across the world, the Toymaker orders his sadistic factotum Stefan to fetch the Doctor for the final test...
The Doctor, Peri and Kevin are brought dinner by the elderly Shardlow, who has been forced to serve the Toymaker ever since losing a game of backgammon in 1776; now that the Toymaker's great work is nearly complete, Shardlow knows that Stefan will challenge him to one last game. The infuriated Doctor finishes his work on the machine which he hopes will open the cell door; the Toymaker has already demonstrated that it is a force-field controlled by his mind, but the Doctor believes that the Toymaker would not waste energy on maintaining the field indefinitely, and simply uses a telepathic control to switch the force field on and off. Unfortunately, his machine fails and simply interconnects the cells, thus putting the Doctor, Peri and Kevin in touch with the Toymaker's other alien prisoners, including the Ventusian Mechanic whom the Doctor met earlier. While their fellow prisoner, a cyborg warrior, lapses into nostalgic reminiscences about the wars it's fought in the past, the Doctor and the Mechanic manage to communicate and lay plans for a new machine which should get them out of the cells. Stefan arrives before they can set to work, and the Doctor, telling Peri to give him a yell if she needs him, sets off to play the game; however, he's too late to save Shardlow, who has already played his last game of backgammon with Stefan.
The Toymaker reveals that he's been on Earth for quite some time, having come to appreciate the obsession the human race has with games. The Doctor agrees to play the Great Game when the Toymaker threatens to turn Peri over to Stefan as a plaything, and as he plays he comes to realize that the Toymaker has been kidnapping random humans from the funfair and analysing their biology in order to ensure that the Game is as realistic as possible. He also realizes that the game portrays a single player, lost and alone in a hostile alien world -- and comes to understand that the Toymaker is alone in this Universe, cast out from his own by some unimaginable catastrophe. Since he is not from this Universe he obeys a different set of physical laws, and since the two Universes are receding from each other, the laws of relativity dictate that his own life span is increasing. The Toymaker is already millions of years old, and he will live for millions more, and the isolation and loneliness have driven him mad; he long ago abandoned the empty thrills of creation and destruction, and now seeks distraction in the random hazards of gameplay.
The Mechanic wires circuitry from the cyborg warrior into the Doctor's machine to build a new device which it attaches to Peri, but she's uncertain how to use it. Out of desperation, the Mechanic attacks her, and as she struggles she cries out for the Doctor. Her voice echoes in the Toymaker's mind, disorienting him just as one of the video game monsters is about to kill the Doctor. In the confusion, the Doctor is able to overpower Stefan and throw him into the path of the monster, which kills him. The monster vanishes when the Toymaker loses consciousness, and the Doctor rushes to the cells, collects Kevin and Peri, and takes them to search the Toymaker's office. There, he locates the telepathic unit which the Toymaker had used to control the force fields around their cells, and rewires it. When the Toymaker regains consciousness he finds himself sealed in a force field maintained by his own thoughts; he will remain trapped there for as long as he lives. The Doctor is upset with himself for taking such drastic action, but realizes that he had no choice; if the Toymaker had been allowed to complete his work, the deadly video games would have been distributed across the world and killed millions. Kevin is reunited with his brother, and the Doctor advises them to use the blueprints for the other, non-deadly video games, to start their own company. He and Peri then return to the funfair.
Source: Cameron Dixon