The Masque of Mandragora

From http://www.bbc.co.uk/doctorwho/classic/episodeguide

The TARDIS is temporarily captured by the Mandragora Helix, a spiral of energy with a controlling influence, at the centre of which the ship is infiltrated by a sparkling ball of energy.

The travellers then move on to the Dukedom of San Martino in Renaissance Italy, where the Doctor quickly realises that the Mandragora energy is loose. The energy enters an underground temple and reveals itself to the outlawed Brotherhood of Demnos, whose leader, the court astrologer Hieronymous, is instructed to make ready for Mandragora's full appearance.

Hieronymous is a pawn in Count Federico's schemes to usurp his young nephew Giuliano, whose accession to the Dukedom is being marked with a celebratory masque.

At the height of the ball, the Brethren attack the court and kill many guests. Hieronymous, now completely absorbed by Mandragora, confronts the Doctor in the underground temple and attempts to blast him down.

The Doctor, however, has earthed both himself and the altar so that the energy simply drains away, leaving the planet safe - at least until the constellations are again in the correct configuration for the Helix to make contact.

2 out of 86 (2.3%) raters say this story requires a previous story.