Drawn by an unknown force, the Doctor's Tardis is brought to a station in space. Oddly, he no longer has Peri with him. Seeing the only door available to him, he walks into a courtroom. The High Council of Gallifrey is putting the Doctor on trial once again at the behest of the Valeyard, a character bent on finding the Doctor guilty of serious crimes of intervening in other planets affairs. The Doctor claims that as President of Gallifrey, he can't be put on trial. He's then informed because he refused to return to Gallifrey, he was deposed. Reluctantly, the Doctor sits down, allowing the trial to continue.
The Valeyard is utilizing the Matrix to illustrate the point of how the Doctor willingly gets over involved in the affairs of others. Using a visualizer in the courtroom, we see the Doctor and Peri materialize on a mysterious planet. The Doctor says the planet is named Ravalox and was to have been destroyed by a fireball, but it's evidently not true. To add further mystery, it's the same size and angle as the Earth. As they continue to investigate, they are being watched by a pair of rouges who want to kill the Doctor, claiming they hate 'competitors'. The man in charge is Sabalom Glitz, a mercenary.
As the matrix continues to follow Glitz and his companion, it bleeps out phrases of their speech to which the Doctor cries fowl. The Inquisitor allows the edited version for now, turning back to the visualizer. The Doctor and Peri have split up when they discover an abandoned "Marble Arch" underground station, which seems to confirm to the Doctor this is actually the planet Earth. As he continues to investigate Peri is captured by a local tribe who have also captured Sabalom Glitz and partner. Meanwhile, the Doctor is captured by the underground dwellers who think he's stealing water. After being stoned, he manages to escape and is also captured by the outside tribe. But there is an 'immortal' living below who wants the Doctor recaptured so he sends a L1 robot to recapture him. While the Doctor is captured, Peri and Glitz are able to escape.
While underground, the Doctor meets Drathro, a robot designed to help maintain order in the facility, but has gone a little mental. And when the Doctor tries to tell it, a faulty black light converter needs to be shut down before there is a cosmos shattering explosion, Drathro refuses to let him do so, realizing that means it will cease to function.
After destroying the L1 robot, Katryca, leads the rest of her tribe into the underground station to steal the secrets within. They believe the service robot was in fact the immortal. Before too long, she realizes her mistake when she storms into Drathro's lair where he kills her by electrocution. Drathro orders everyone else outside to awate being 'culled'. They obediently do as they are told. This gives the Doctor one last chance to disable the converter, but still fails to do so.
Trying once more, this time with Glitz' help, the Doctor is able to stop the catastrophe. Glitz has tricked Drathro into thinking he can support it with black light from his ship. Realistically though, Glitz was trying to steal away the secrets Drathro was carrying. When the Doctor averts most of the explosion, Drathro is destroyed, taking the secrets Glitz so desperately wanted. The crisis is averted, and back in the courtroom, the Doctor gleefully says he's won the case because he averted the destruction of the universe. But the Valeyard contents his meddling possible initiated it and has one more case to show the court.
Bonus Photo: The Valeyard
Who is the Valeyard, and why is he so determined to see the Doctors demise?