Philip heads to Earth in pursuit of a Fifth Columnist and his pregnant wife, who have stolen the Mothership's laser power source. Philip also determines to kill Mike Donovan, whom he believes is responsible for the torture and death of his brother Martin.

The alien space station J7, the mid-1980s Earth time. The Time Lords have sent the second Doctor and Jamie here to try to stop dangerous experiments into time travel. But the station comes under attack from the Sontarans and far away in space and time the sixth Doctor goes into psychic shock...