As plague sweeps 17th century England, the Doctor discovers a more exotic threat at work - shipwrecked Terileptil convicts, intent on making the planet their own.

To rescue Tegan and Adric, captured and interrogated by the Terileptil fugitive leader, the Doctor must get past the locals under alien control and overcome Death (an android servant dressed up as The Grim Reaper).

The Doctor makes the fugitive Terileptils a rare offer they shouldn't refuse; but, though few in numbers, they have a plan in the works for the quick and complete domination of Earth and mean to see it through.

With Tegan and Richard under alien control, and the manor sealed against anyone leaving or entering, it's a question whether or not the Doctor can get to London before the Terileptils release their improved strain of black death upon the world.