Parts: 4
Writer: Louis Marks
Airdates: 4 September 1976 - 25 September 1976
Production Code: 4M

Doctor: fourth
Companion: Sarah

Creature Feature: Mandragora Helix

Story: The TARDIS gets caught in the pull of the Mandragora Helix, a gigantic energy spiral in space. After materializing inside the Helix, the Doctor and Sarah take a quick look around, but a near-encounter with deadly Helix energy persuades them that leaving is the best course of action.

They next materialize in the Italian town of San Martino, sometime in the late fifteenth century. The new duke, Giuliano, is under siege from the evil count Federico and court astrologer Hieronymus - Giuliano's father died by their hand, and Federico intends for Giuliano to be next, leaving him as duke. And in the dark catacombs under the city, the malevolent Cult of Demnos is biding its time to seize power and control the world.

An innocent orange-picking jaunt delivers Sarah into the hands of the Brothers of Demnos, who intend to sacrifice her to their god. When the Doctor goes looking for her, he comes across something even more horrible - a ball of Helix energy, transported to Earth in the TARDIS, that could enslave or destroy all of mankind...