After an encounter with the deadly Mandragora Helix, Tom Baker as the Doctor and Sarah Jane land in 15th Century San Martino. In the midst of danger, secrecy and intrigue, they witness the flowering of the Italian Renaissance. As the masque to celebrate the accession of the new Duke approaches, the Doctor realises that a third visitor has arrived with him in the TARDIS. It is a force with the power to wipe out human civilisation forever. The Doctor has brought it to Earth – and only the Doctor can stop it...