The Doctor, attempting to return Tegan to Earth, arrives three hundred years too early. After an attack by superstitious villagers, the Doctor's party escape and find temporary refuge in the company of actor-turned-highwayman Richard Mace. In a barn the Doctor locates a power cell of alien origin. A group of Terileptils have crash landed on the Earth, and now they are developing a diabolical plot to wipe out humanity. The Doctor must stop the Terileptils before it is too late.