From http://www.bbc.co.uk/doctorwho/classic/episodeguide:
The TARDIS materialises in London sometime after the year 2164. Dalek invaders are now ruling the Earth with the aid of humans converted into zombie-like Robomen, but they are opposed by a group of resistance fighters led by the wheelchair-using Dortmun.

The travellers discover that the Daleks have established a huge mine in Bedfordshire, their aim being to remove the Earth's core using a huge bomb and replace it with a powerful drive system so that they can pilot the planet around the galaxy. Ian manages to create a barrier in the shaft in order to intercept the bomb. The resulting explosion destroys the Daleks and their mine and creates a huge volcanic eruption.

Susan has fallen in love with resistance fighter David Campbell, and the Doctor decides to leave her on Earth to find a new life with him, while he continues on his travels with Ian and Barbara.