The Ultimate Foe

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

With the evidence complete, the Doctor learns that the Master has gained illicit access to the Matrix in his TARDIS. Glitz is now revealed to be the Master's associate and the 'secrets' to be information stolen from the Matrix.

The Valeyard admits his identity as a distillation of the dark side of the Doctor's nature, somewhere between his twelfth and thirteenth incarnations, out to take control over his remaining lives.

With the help of Mel, who along with Glitz has been brought to the space station by the Master, the Doctor defeats his future self - although, as they leave in the TARDIS with all charges in the trial having been dropped, it appears that the Valeyard has taken over the body of the Keeper of the Matrix and may not have been as completely vanquished as they had thought...

49 out of 54 (90.7%) raters say this story requires a previous story.