Materialising in outer space, the TARDIS is attacked by a missile fired from the dark side of the moon.

Back on Earth, the newly-formed United Nations Intelligence Taskforce, led by Brigadier Lethbridge-Stewart, is disturbed by a series of UFO sightings over Southern England.

Meanwhile, a large consignment of mysterious crates is delivered to the headquarters of International Electromatix, the largest computer and electronics firm in the world.

Three seemingly unconnected events — but in reality the preparations for a massive Cyberman invasion of Earth with one aim — the total annihilation of the human race.