The Doctor and Jo have gone off in the TARDIS, leaving the Brigadier and UNIT facing a deadly mystery — and a moral dilemma...

Robbery and murder are on the increase in Britain as disputes between underworld gangs escalate into open warfare on the streets. The Master seems inextricably linked to the chaos — despite the fact he is safely under lock and key.

Meanwhile UNIT is called in when a plane missing in strange circumstances is rediscovered — contaminated with radiation and particle damage that cannot possibly have occurred on Earth.

As the mystery deepens, what little light they can shed on the matter leads the Brigadier to believe that with the Doctor away, Earth's only hope may lie with its greatest enemy...