Synopsis: The Doctor shelters from an attack on his mind in the TARDIS where he identifies the source as being an alien spacecraft on the fringes of the solar system. Travelling to London, the Doctor intends to warn the Minister of Defence about the imminent invasion. However, the Doctor discovers the minds of the entire population of Earth have been neutralised by the approaching Zeron invaders. Teaming up with an unaffected American tourist called Nick, the Doctor uses the resources of the Post Office tower to counteract the Zeron mind control. Fearful of looking foolish if the Doctor should be wrong, the British Government refuse to alert the world to the threat of the Zeron invasion. With the help of Nick's disc jockey cousin Jed, the Doctor broadcasts his own warning over the radio. As the Zerons once again bombard the Earth with their mind-control ray, the Doctor leads his friends to the safe depths of the London Underground. However, the Zerons are aware of the Doctor's refuge and take steps to neutralise the threat he poses towards their conquest of Earth.