The TARDIS materialises in London in mid-22nd century. 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. The Doctor and his companions discover that the Daleks have created a large mine in the English countryside for the purpose of removing the Earth's core (using a massive explosive device) and replacing 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 and deflect the bomb. The resulting explosion destroys the Daleks and their mine. The Earth is safe once again! However, 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. The Doctor continues on his travels with Ian and Barbara.