The Doctor and Sarah are investigating some unfamiliar areas of the TARDIS and come across the secondary control room, wood panelled and with a smaller central control console. Switching on the scanner, the Doctor realises that they are coming close to the Mandragora Helix, a spiral of energy radiating outwards with a controlling influence at the centre.

The TARDIS is captured by the Helix and drawn to a still point at its centre. After a brief look outside, the Doctor dematerialises the ship, but a sparkling piece of Mandragora energy has entered the TARDIS and emerges at the next point of call, Earth towards the end of the 15th Century.

The Doctor and Sarah find that they are in Italy in the Dukedom of San Martino and realise that the Mandragora energy is loose and up to no good.

The energy enters an underground temple where the outlawed star worshipping Brotherhood of Demnos gathers. It reveals itself to them during one of their ceremonies. The worshippers, led by the court astrologer Hieronymous, are awed and Hieronymous receives instructions to make ready for Mandragora's full appearance.

Hieronymous is one of the pawns in the evil Count Federico's plans to take control of the Dukedom for himself. He has already arranged for the old Duke to die under 'mysterious circumstances' as predicted by the stars, and is planning for the same to happen to the new Duke, Giuliano. Hieronymous, increasingly under the influence of Mandragora, realises that the Doctor is a threat and hypnotises Sarah with instructions to kill him.

The Doctor becomes aware of the trap when Sarah innocently asks how she can speak and understand the local language - something that had not previously bothered her in all the times and places she had visited. He de-hypnotises her and makes arrangements to remove Mandragora's influence from Earth. At the height of a masked ball, the Brethren of Demnos attack the court, and Hieronymous, who has now been completely taken over by Mandragora, confronts the Doctor in the underground temple. The Doctor has earthed himself and the altar so that as the Helix energy blasts him it is carried safely away. In this way he drains all the Mandragora energy on Earth, leaving the planet safe, at least until the constellations are again in the correct configuration for the Helix to reestablish contact.

_{Synopsis from Doctor Who: The Fourth Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}