The Deadly Assassin ( TV story ) The Deadly Assassin was the third story of season 14 of " Doctor Who " . It was the only televised story in the original run of " Doctor Who " to feature the Doctor without a companion . Tom Baker had told Philip Hinchcliffe he could hold the show on his own . With this story already in place , it was seen as a pilot for such companion - less stories . However , it was deemed that a companion was a necessary feature of the show . This serial saw the return of the Doctor ' s nemesis , , but in a heavily decayed state played by Peter Pratt . Roger Delgado had died in an automobile accident three years earlier , requiring a new actor to take his place . Afterward , the Master became subject to a change in appearance and other changes as needed when an actor replaced the role , much like the Doctor . Narratively , this serial introduced several aspects of Time Lord society which were used or referenced again , including the Matrix , Time Lord Chapters , Time Lord headdresses and robes , and of course Rassilon . It also introduced the restriction of a Time Lord to a finite limit of twelve regeneration s , allowing a maximum of thirteen incarnation s , after which a Time Lord would suffer permanent death . Later stories revealed that there are exceptions to the rule , such as the Master , who , after exhausting his original cycle , stealing a non - Gallifreyan body ( TV : " The Keeper of Traken " ) and being executed by the Daleks , ( TV : " Doctor Who " ) was resurrected by the Time Lords to fight in the Last Great Time War , gaining a new cycle in the process . ( TV : " Utopia " , " The Sound of Drums " , " The Doctor Falls " ) , and the Doctor , who was granted a new regeneration cycle when his final incarnation reached the point of death . ( TV : " The Time of the Doctor " ) Synopsis . Through the millennia , the Time Lord s of Gallifrey led a life of peace and ordered calm , protected against all threats from lesser civilisations by their great power . But this was to change . Suddenly and terribly , the Time Lords faced the most dangerous crisis in their long history . . . Plot . Part one . Having dropped off former companion Sarah Jane Smith back home , the Fourth Doctor heads to Gallifrey in answer to the Time Lord s ' summons . On the way , he is struck by a premonition in which he seems to assassinate the Time Lord President from a gallery overlooking the Panopticon . The TARDIS lands in the security area of the Citadel . Commander Hilred immediately impounds it and orders the arrest of its owner . The Doctor leaves a note on the console warning of his premonition and sneaks out of the TARDIS into the Citadel . He is cornered by a guard , who is shot dead by an unknown assailant . The arrival of an unregistered TARDIS in a high - security area raises the tension of an already tense day — the President is resigning and is about to name his successor . The Castellan Spandrell berates Hildred for his incompetence in letting the Doctor , a renegade who apparently is also a murder er , run loose in the capitol . Hildred transducts the TARDIS into the capitol , unaware the Doctor has sneaked back and hidden himself on board . Meanwhile , his movements are being monitored by a dark , robed figure and an unknown associate . The Doctor infiltrates the resignation announcement by stealing a Time Lord ' s ceremonial robes . While trying to remain incognito in the crowded floor , he encounters an old classmate , Runcible , now a newscaster , preparing his broadcast from the Panopticon floor . Runcible greets him coolly while waiting for a signal from a camera operator in the gallery . The Doctor looks up and is horrified to see a staser rifle fixed to the railing near the unattended camera . He causes a commotion as he charges through the room . As the President enters and stands at the dais , the Doctor grabs the staser rifle . He aims and fires . The President falls down dead . . . Part two . The Doctor is quickly apprehended by security . The assassination has thrown Gallifrey into a constitutional crisis because the President had yet to name his successor . Chancellor Goth , thought to have been the most likely successor , calls for prompt elections and opts to stand as a candidate . Goth also urges the Doctor ' s swift trial and execution . At the trial , Goth ' s prosecution moves swiftly . The Doctor , however , invokes Article 17 of the Gallifreyan Constitution , naming himself as a candidate for President . Under it , he cannot be denied the right to make his claim . Goth is outraged , but Chancellor Borusa ( the Doctor ' s former teacher at the Academy ) acknowledges that the article gives him protection . He is grudgingly given forty - eight hours to prove his innocence . The robed figure is told by his associate of the Doctor ' s use of the constitutional loophole . He has anticipated this . The figure is shown as a horribly disfigured and decaying husk . The Doctor attempts to convince Spandrell and Coordinator Engin of his innocence ; his shot was intended for the actual assassin , who stood in the crowd on the Panopticon floor . Someone is going to great lengths to frame him . He notes that the sights had been fixed on the rifle to intentionally throw off his aim . Spandrell confirms this by aiming at a target himself . He begins to believe the Doctor . They find the Doctor ' s original blast mark on the wall . The Doctor realises the gallery camera would have recorded the actual assassin . Runcible screams with horror when he looks into the camera barrel . Running to the gallery , they find the camera barrel empty except for the miniaturised corpse of the cameraman . The Doctor recognises this as the work of his arch enemy , , and reasons that he has returned to Gallifrey for a final showdown . Runcible goes to fetch the recordings , but when he returns , he falls with a knife protruding from his back . Spandrell and Engin cannot comprehend why there is no bio data extract for the Master in the APC Net ( aka the Matrix ) . This is a network of past and present Time Lord minds that acts as an enormous database and future forecaster . The Doctor decides there must be an unauthorised second access point into the Matrix . The Master used this to forecast the assassination into his mind and then wipe all trace from the Matrix . He reasons that either the Master or the assassin working with him must be inside the Matrix . Despite the stern warning from Engin , he interfaces with the Matrix to find him . The Doctor finds himself in a vast , rapidly shifting terrain , the domain of the assassin . The two engage in a pitched battle of wills . The assassin has the definite advantage of having created the virtual reality world inside the Matrix . The Doctor finds himself in a jungle . He steps on a train track , and it becomes stuck . He looks forward to see a masked man driving a train towards him . . . Part three . The Doctor prepares for impact , but nothing happens . He gets his foot out and moves on . The Doctor evades the many pitfalls laid for him inside the Matrix . These include being strafed by a biplane and tracked by the assassin . His physical body ( still in the APC room ) is enduring a terrible and potentially lethal strain . Meanwhile , the assassin is finding the battle of wills extremely taxing as well . The Master increases the power , despite the assassin ' s plea it will kill him . The Doctor begins to turn the tables on his assailant , first by booby trapping the hunter ' s equipment , then by avoiding the water poison ed by the assassin . He improvises a blowpipe and shoots a poisoned dart at the assassin , but he is wounded himself . As the Doctor comes closer to winning the conflict , the Master sends one of the chancellor ' s guards now under his power to the APC room to kill the Doctor . Engin spots the guard , Solis , tampering with the controls . Spandrell shoots Solis to protect the Doctor . In the Matrix , the Doctor gains the upper hand against the assassin , who reveals himself as Goth . The Doctor tricks Goth into firing his rifle while in a cloud of swamp gas . As the world around them erupts in chaos and flames , Goth seizes the Doctor and holds his head under water , about to drown him . Part four . The Doctor throws him off and escapes from the Matrix . He revives in Spandrell ' s office . He informs the shocked Castellan of the assassin ' s identity . They trace the location of their lair , where they find the Master ' s lifeless body — he seems to have died of natural causes . Goth , himself near death , admits he was power - hungry and bitter on learning he wasn ' t to be the President ' s successor . He had found the dying Master on planet Tersurus , his body at the end of his regeneration cycle , and brought him to Gallifrey to help him fulfil his scheme . Goth dies before he can reveal just what the Master ' s plan was . Cleared of all charges , the Doctor still has lingering doubts . He wants to know the Master ' s plan . He doubts the Master would accept death so easily . He reasons that the solution lies in the ceremonial relics given to the President on induction , the Sash and Rod of Rassilon , and researches their links to ancient Gallifreyan mythology . The Doctor ' s suspicions are confirmed . The Master has faked his own death . He steals the Sash and Rod , which are the keys to the Eye of Harmony , the heart of a black hole captured by ancient Time Lord Rassilon . It is the source of Time Lord power . The Master seeks the power of the Eye to restart his regeneration cycle , even though Gallifrey would be destroyed by doing so . He uses the Rod to unlock the Eye of Harmony , hidden below the Panopticon floor . This begins to release its energy , which would be channelled through the Sash to rejuvenate him . The Doctor wrestles with him . The ground shakes around them . Before the Master can uncouple the last cable from the Eye , the Doctor pulls him away , and he falls through a fissure in the floor . The Doctor reconnects the cables , bringing the crisis to an end . Borusa is appalled at the damage ; half the capitol city lies in ruins and countless lives are lost . Even so , he accepts Engin ' s claim that the Doctor ' s actions prevented further catastrophe . Recalling their old relationship as teacher and student , Borusa gives the Doctor a grade of 9 out of 10 . The Doctor departs in the TARDIS . Afterwards , Spandrell discovers that the Master has survived and escaped in his own TARDIS , disguised as a grandfather clock . He expresses confidence that the Doctor and the Master will cross paths again . Before the Master ' s TARDIS dematerialises , the face of the grandfather clock is replaced by that of the Master . . . Home video and audio releases . DVD releases . Special Features include :