The survivors of a wrecked ship are stranded with the lighhouse crew on Fang Rock as a deadly menace slithers through the fog. Has the Beast of Fang Rock returned?

The Doctor barricades everyone in the lighthouse to protect them from the mysterious deadly creature in the fog. But it may already be inside, biding its time…

The Rutan starts picking off the surviving humans one by one as the Doctor and Leela struggle to defeat not only the alien but an entire fleet waiting to occupy Earth.