An attempt to return to Tegan to Heathrow Airport fails, and the Doctor and his companions arrive in 1666 England in the darkest days of the Great Plague. As they venture into a land gripped by fear and mistrust, they soon discover that they are not the only new arrivals. Strange lights have been seen in the sky, the Grim Reaper stalks the local woods, and evidence of an advanced alien technology is all around.