The Ultimate Foe ( TV story ) The Ultimate Foe was the unbroadcast title given to episodes 13 and 14 , the concluding episodes of " The Trial of a Time Lord " , the series - long storyline that constituted Season 23 of " Doctor Who " . The on - screen title was simply " The Trial of a Time Lord " . This story marked the final televised appearance of Colin Baker as the Sixth Doctor . After much controversy had surrounded the Sixth Doctor ' s era , the BBC decided to recast the Doctor the following year to start with a clean slate , ending Baker ' s tenure prematurely . As a result of Baker ' s removal from the role the next episode would begin with the sudden regeneration of the Sixth Doctor into the Seventh Doctor , Colin Baker himself not returning to film his Doctor ' s regeneration scene . In addition , this story marked the final appearances of the Inquisitor and the Valeyard as recurring characters , and a final appearance of Nicola Bryant as Peri Brown in a cameo to give her character closure under happier circumstances than what the events of " Mindwarp " had offered . This was also the last of the Master ' s annual appearances on the show ; he wouldn ' t appear again until " Survival " in Season 26 . Finally , the conclusion of Season 23 offered a significant revelation which gave insight on the possible origin of the Valeyard . However , after Colin Baker was dismissed from the series , the story arc between the Sixth Doctor and the Valeyard was dropped . Decades later , Big Finish Productions would resume the arc in audio story format . Season 23 ' s finale was the final story to which longtime scriptwriter Robert Holmes contributed . He passed away from illness shortly before its completion . Unfortunately , this would cause the writing process to suffer many setbacks before entering production . The sudden demise of Holmes served as the catalyst for a notorious fallout between script editor Eric Saward and then - executive producer John Nathan - Turner . Saward , now left without Holmes , had to complete the last episode by himself . Nathan - Turner , however , rejected his script , who felt Saward ' s proposed cliffhanger was presented in a way that would encourage Michael Grade , the BBC controller at the time , to make it into a series finale , after Grade had already tried to cancel the series altogether . The Doctor and the Valeyard would have tumbled through the Matrix , fighting to the death , with the battle ' s outcome left unknown , but with the assumption that they would be locked in eternal combat if no one intervened . Interestingly , a plot similar to this was depicted in 2015 for the Sixth Doctor ' s long - awaited regeneration story , AUDIO : " The Brink of Death " . Not the first time Saward had butted heads with Nathan - Turner over creative direction , his aggravation was enough this time that he chose to resign from his position , banning use of his scripted ending in further dissent , effectively making this his last contribution to the televised series . The conclusion of the story ultimately fell in the hands of writing couple Pip & Jane Baker , who were left to figure out an ending of their own . They were prohibited access to the original script and given no bearing on how the story was meant to end , but still did what they could to wrap up the loose ends and encourage the continuation of the classic series a little longer . Synopsis . Charged with genocide by the treacherous Valeyard at his trial , the Doctor receives help from an unlikely source to turn the tide of the High Council ' s rulings in his favour and reveal the Valeyard as a wrongdoer - the Master . For the Valeyard ' s own crimes are so atrocious , even the Doctor ' s archenemy will help him to ensure that the villain won ' t see the light of day again . Cornered , the Valeyard flees to the Matrix , where he can be the Doctor ' s judge , jury and executioner . . . Plot . Part thirteen . The Sixth Doctor insists that the footage from the Matrix has been tampered with . The Inquisitor brings the Keeper of the Matrix to testify . He is adamant that the Matrix can be accessed only by senior Time Lord s with appropriate keys . The Doctor maintains his innocence , accusing the Valeyard of manipulating the evidence to his own ends and that someone " can " make a duplicate key . The Valeyard denies any such interference and closes his case . Meanwhile , two travel pods arrive on the station . They open to reveal Mel and Sabalom Glitz . They enter the court just in time to assist the Doctor ' s defence , saying they had been sent by someone unknown to help prove that the Doctor acted in good faith . This anonymous benefactor makes himself known , appearing on the viewscreen from inside the Matrix - it is the Master . He entered the Matrix with a duplicate key and has been watching the courtroom drama unfold but is , for some reason , unwilling to let the Valeyard win . The Doctor questions Glitz about the secrets he was hoping to obtain from the sleepers . He learns those secrets were stolen from the Matrix . In retaliation , the Gallifrey an High Council moved Earth to hide the theft and prevent a rescue . The Doctor is outraged at the corruption of his own people , realising that he has been framed to prevent the truth from emerging . The Master adds that the Valeyard is an amalgamation of the Doctor ' s darker impulses from the future , falling somewhere between his twelfth and final incarnations . The High Council offered him the Doctor ' s remaining regeneration s if he could convict the Doctor . The Inquisitor insists the trial consider all the evidence , but the Valeyard flees the court through a door into the Matrix . The Doctor and Glitz follow through the door , finding themselves in a recreation of Victorian London created by the Valeyard . The Doctor is attacked by a hand from a rain barrel , but Glitz saves him and hands him a note from the Master which leads them to the Fantasy Factory . As they approach , Glitz is shot with a harpoon . In the courtroom , the Master explains that the evidence presented throughout the trial was mostly correct , but with small errors designed to convict the Doctor . These included the death of Peri , who actually survived to become Yrcanos ' queen . He admits that the Valeyard would make an even more powerful enemy than the Doctor , but this way he could be rid of them both . He also insists that the High Council answer for what they have done and has allowed the people of Gallifrey to witness the court proceedings . Glitz , saved by his Mark 7 postidion life preserver , accompanies the Doctor to the Fantasy Factory , where they encounter an extremely officious bureaucrat named Mr . Popplewick . Deciding to go over his head and speak directly to the proprietor , they march into the next room , only to find an identical office with an even more evasive and infuriating duplicate of Mr . Popplewick . Before letting them proceed further , Popplewick asks the Doctor to sign a document that promises his future incarnations to the Valeyard should he vanquish the Doctor , the High Council being less than trustworthy and unlikely to keep their promise . As the Doctor steps through the next door , he finds himself alone on a beach , where the Valeyard ' s voice taunts him and hands emerge from the sand to pull him beneath the surface . . . Part fourteen . Glitz arrives as the Doctor is dragged under , only grabbing the cloth around each of his shoes . He is amazed to see the Doctor rise out of the ground unharmed , after making a bad pun . The Doctor explains that the Matrix is unreal and that , with enough effort , he can deny the Valeyard ' s traps . The Valeyard appears , taunting the Doctor . He explains that he has to destroy the Doctor ' s good side to be free of all his positive traits . A cloud of nerve gas advances towards them , forcing the Doctor and Glitz to take refuge in a nearby beach hut that turns out to be the Master ' s TARDIS . The Master explains the Valeyard has to be stopped because he has none of the Doctor ' s morality , making him an even more evil being than himself , which vexes the Master . Lying that he wishes to help the Doctor , the Master tricks the Doctor into believing Glitz and he are retrieving his TCE from elsewhere in the TARDIS , but activates a function on his console that puts the Doctor into a catatonic state while Glitz and he hide in the corridor . The Master ' s TARDIS materialises at the Fantasy Factory and the hypnotised Doctor is sent out as bait . When the Valeyard comes out to see what is happening , the Master shoots the Valeyard with his TCE , but the beams from his weapon bounce off . The Valeyard retaliates with explosive quills that force the Master to run away . Glitz is temporarily stunned by an explosion . Mel arrives in the Matrix to help the Doctor . They return to the station to finish clearing his name . Mel gives evidence regarding the Vervoid s , but it is not enough to prevent the Inquisitor delivering a verdict of guilty on the charge of genocide . The Doctor accepts his death sentence with surprising calm . This is not the real courtroom , but another Matrix fantasy . On the real station , Mel and the Inquisitor watch impotently as the Doctor is taken to what he believes to be his execution . Mel is unwilling to sit by meekly . She steals the Keeper ' s key to enter the Matrix . She reaches the Doctor in time to save him , but he is well aware of the situation because the fake Mel had mentioned events she had not witnessed . The Doctor was hoping to encounter the Valeyard , so they head for the Fantasy Factory . The Master , back in his TARDIS , tries to hypnotise Glitz into helping him , but has to resort to bribery when the hypnotism fails . This proves just as ( or possibly even more ) effective . Glitz finds the Matrix tapes containing the secrets in Popplewick ' s office , while the Doctor locates a list of the courtroom judges written in his own handwriting . Glitz forces Popplewick at gunpoint to take them to the proprietor , J . J . Chambers , but is willing to trade the Doctor for the Matrix secrets , which he then gives to the Master . The Doctor exposes Popplewick as the Valeyard in disguise , since his melodramatic nature was too obvious . He finds a laser aimed through the viewscreen into the courtroom , to kill all the judges on the list as a last resort . The Master reveals to the court that the High Council has been deposed by a revolt on Gallifrey and he intends to rule in their place . He loads the tapes of the secrets into his TARDIS console , but it is a fake which freezes the Master and Glitz in the Matrix . Mel arrives in the courtroom in time to evacuate the judges , while the Doctor stops the laser firing at the cost of creating a massive feedback surge which strikes the Valeyard , allowing the Doctor to escape back to the station . The Inquisitor dissolves the trial and tells the Doctor about Peri ' s true fate on Krontep . She further suggests that the Doctor , for the third time , run for the vacant presidency on Gallifrey , but he declines , stating that she would be a better candidate . He suggests that , while the Master must be punished , leniency should be shown to Glitz as he can be reformed . The Doctor and Mel depart in the TARDIS . She annoys him by mentioning carrot juice . The Doctor almost barrels off in the opposite direction once he learns she plans to make him exercise again , thinking he would have been better off taking the Presidency . However , he gives in to her whims when he remembers Mel has met him too early . He intends to return her to whence she came and wait to meet her in the original order . Back in the court room , the Inquisitor orders the Keeper to improve the security of the Matrix and repair it while removing the Master and Glitz . He is allowed to requisition anything necessary for its mending . The Keeper agrees but , as he turns away , he reveals himself to be the Valeyard . . . Story notes . Production errors . - A great deal is made about the fact that Mr Popplewick keeps a quill behind his ear , but the quill only appears when it is mentioned . Home video and audio releases . DVD release DVD Contents VHS releases