The Doctor fights the Valeyard in the Matrix

Tags: Sixth Doctor, The Valeyard

The Trial of a Time Lord: The Ultimate Foe, episode 14
16 December 1986

With the revelation that the Valeyard is a projection of the Doctor’s darker side, he flees into the Gallifreyan Matrix. The Doctor and Glitz follow him, but within the supercomputer, things are decidedly surreal...