'There is a old Venetian saying,' the Doctor murmured. 'The council of ten send you to the torture chamber; the council of three send you to the grave.'

A strange invitation brings the Doctor, Steven and Vicki to Venice in the year of our Lord 1609: a place of politics and poison, science and superstition, telescopes and terror. Galileo Galilei is there demonstrating his new invention to the Doge, and William Shakespeare is working as a spy for King James I. And there are other visitors too: inhuman ones that lurk in the shadows, watching - and killing.

Vicki is abducted to a flying island. Steven is accused of murder and challenged to a duel. The Doctor, meanwhile, finds himself at the centre of what looks like an attempted invasion. But who are the invaders? And why can’t they proceed without his help?