Fifty years before the arrival of the TARDIS crew, Earth decided to colonize the planet Mechanus. Probably owing to the vicious vegetation of the planet, such as large fungoid creatures with bells that descend on their prey, Earth decided to begin colonization with a rocketful of robots programmed to clear landing sites. Presumably these robots - known as Mechanoids - were also programmed to begin construction on a city for the colonists to inhabit. But before immigration could begin, an interplanetary war broke out. Earth's attention became focused elsewhere, and Mechanus was forgotten.
The Mechanoids are very tall and wide spherical robots covered in triangle-shaped panels. The top of the sphere can extend upward, exposing some kind of dangling machinery whose purpose is unknown, although it may have something to do with communication. Two mechanical arms fold out from the center of the sphere, and the Mechanoids are also equipped with a powerful flame gun, which they will use at the first sign of violence.
Along with the city, which was built 1,500 feet above the top of the jungle, the Mechanoids constructed a lift system to the surface of Mechanus. They also built a corridor of lights to show the way to the lift and frighten away the light-sensitive fungal creatures. But the Mechanoids are only programmed to respond to the Earth colonists, who know the Mechanoids' code. Any visitors to the city who do not respond properly are practically held prisoner and displayed like specimens in a zoo. Despite being robots, the Mechanoids are perfectly capable of waiting for the colonists to arrive, because they are programmed to self-repair.
The Mechanoids' peaceful existence continues uninterrupted until the Daleks visit the planet in pursuit of the TARDIS crew. Their hostile manner makes them immediate enemies of the Mechanoids, and a war breaks out, with both sides causing numerous casualties. Meanwhile, a device created by the Doctor to destroy the Daleks explodes, setting off a fire which eventually destroys the Mechanoids' city.
