Arc of Infinity ( TV story ) Arc of Infinity was the first story of the " Doctor Who " anniversary season 20 . Janet Fielding returned as Tegan Jovanka in this serial , rejoining the Fifth Doctor and Nyssa on their travels . Future Doctor Colin Baker made his " Who " debut in this story , playing Commander Maxil . Just over a year later , he would take over as the Sixth Doctor at the end of season 21 . Of the several " Doctor Who " stories filmed outside the UK in the late 1970s - early 1980s , this was the only one to open a season . Synopsis . Omega , an ancient Time Lord made of pure anti - matter , once defeated by the Doctor , is plotting to cross over into this dimension by bonding with the Doctor . Meanwhile , the disappearance of a man in Amsterdam piques the curiosity of his cousin , Tegan , who previously left the Doctor at Heathrow Airport and now finds herself at Omega ' s mercy . Fearing total destruction from the collision of matter and antimatter , the Time Lords recall the Doctor to Gallifrey to undertake the only viable solution : executing him ! Plot . Part one . On Gallifrey , a Time Lord is at work stealing the biodata extract of another Time Lord and killing a technician who stumbles across the crime . The traitor provides the bio - data to a creature composed of anti - matter and uses the bio - data to invade the TARDIS and then the Fifth Doctor ' s metabolism . The Doctor ' s companion , Nyssa , helps him recover . The creature is shielded in this attempt by the Arc of Infinity , a curious curve between the dimensions containing quad magnetism which can shield anti - matter . The Doctor decides to head to Gallifrey to track down the supplier of his bio - data , conscious that , unless the creature trying to cross universes is stopped , its incursion could cause a fatal chain reaction to our universe . Meanwhile , in Amsterdam , Netherlands , backpackers Robin Stuart and Colin Frazer decide to crash at the crypt of the Frankendael mansion . During the night , Colin hears something and investigates ; he finds a curious birdlike creature , the Ergon , which hypnotises him . A terrified Robin runs away . The High Council of the Time Lords is also taking the matter seriously and has decreed that the Doctor ' s TARDIS should be recalled . The Chancellery Guard under the over - zealous Commander Maxil seizes the Doctor and Nyssa . When the Doctor resists arrest , Maxil shoots him . . . Part two . The Doctor has been stunned to ensure his delivery to the High Council . When he is brought before the High Council the new Lord President , Borusa , is inscrutable while Chancellor Thalia and Cardinal Zorac are openly hostile ; only his old friend Councillor Hedin seems pleased to see him . The President stresses the gravity of the situation since the creature poses such a threat to the Universe , and the High Council has had no alternative but to issue a Warrant of Termination on the Doctor to ensure the creature can no longer bond with him . Fortunately an old friend , Damon , who is another technician in the records section , provides him with the proof he needs that a member of the High Council stole his bio - data extract . Meanwhile , the Doctor ' s former companion , Tegan Jovanka , arrives in Amsterdam looking for her cousin Colin . Instead , she is greeted by Robin , who tells her that Colin has disappeared . When neither of them can persuade the police to take an interest they decide to investigate the crypt themselves . The Doctor is soon taken for execution , despite Nyssa ' s attempts to save him , and placed in a dispersal chamber . Sentence is carried out . . . Part three . The supposed death of the Doctor , however , has not solved the situation . Unbeknownst to the High Council , his mind has been taken into the Time Lord living repository of knowledge , the Matrix , while his body is hidden behind a force shield in the termination cubicle . The creature is a renegade Time Lord , who demands an opportunity to return to the Universe it once inhabited . The truth of the aborted execution is discovered by the wily Castellan , who , while questioning Nyssa and Damon about their involvement , tells them that the Doctor is alive . Castellan summons and then tells the High Council . Tegan and Robin are attacked by the Ergon ' s weapon , a matter converter , and transported into a TARDIS hidden at the Frankendael and belonging to the renegade . Their minds are scanned , revealing to the creature that Tegan knows the Doctor . The renegade uses Tegan as bait to force the Doctor to obey him , also releasing Colin from his slavery as a reward . The Doctor is returned to normal space on Gallifrey where he makes for the High Council Chamber . Lord President Borusa has fallen under suspicion of being a traitor because the Castellan reveals it was his codes that were used to transmit the bio - data . The truth , however , is that Councillor Hedin is the traitor . He is in awe of his master - the mighty Omega , first of the Time Lords and pioneer of time travel ( see " The Three Doctors " ) . Hedin wishes to release Omega from his exile in a universe of anti - matter , not realising the great Time Lord has been driven mad by his years of solitary confinement . The Castellan kills Hedin , but this does not prevent Omega using the Arc of Infinity to seize total control of the Matrix and , therefore , the organisation of Gallifrey . . . Part four . Fortunately the Doctor and Nyssa manage to slip away and return to the TARDIS . They use the scant knowledge provided by Tegan to determine that Omega has established his base in Amsterdam on Earth , and head there immediately , desperately trying to find the Frankendael crypt she described . After a lengthy hunt they find the lair defended by the Ergon , and Nyssa disposes of it with its own matter converter . They reach Omega ' s TARDIS at the point at which both the ship is destroyed and Omega makes full transference to Earth using the Arc of Infinity . When he peels his decayed mask away he reveals the features of the Doctor . Omega heads off into Amsterdam with the Doctor , Tegan and Nyssa in hot pursuit . Within a short time the Doctor ' s prediction of an unstable transfer begins to come true : Omega ' s flesh decays and it is clear his new body is not permanent . When the Doctor , Tegan and Nyssa catch up with him it is a painful task for the Doctor to use the Ergon ' s matter converter on Omega , expelling him back to his own universe of anti - matter . The Time Lord High Council on Gallifrey detects the end of the threat . Once Tegan has checked on her cousin ' s progress in hospital , she decides to rejoin the TARDIS crew . This time it is as a willing traveller . Story notes . Production errors . " to be added " Home video and audio releases . DVD releases . This story was originally released in a double - pack with " Time - Flight " in Region 2 and 4 , but released singly in Region 1 . The R2 and R4 cover art of this story and " Time - Flight " shows the " Peter Davison Years " as 1981 - 1984 . All other Davison - era releases have claimed the years as 1982 - 1984 in deference to the January , 1982 broadcast of " Castrovalva " . However , there is justification for calling the era 1981 - 1984 , as that ' s the period of time Davison actually worked on the programme . Like Jon Pertwee , Davison fell victim to the BBC ' s decision to push back the premiere of his first series to the start of the new calendar year . Neither actor is generally credited for their first year on the job , making their eras appear a little shorter than they actually were . While Pertwee only filmed about half of Season 7 in 1969 , almost everything of Season 19 was filmed in 1981 . Indeed , Davison ' s first work on the series — his regeneration scene — had been filmed on 9 January 1981 , almost a full year prior to the release of " Castrovalva " . Ironically , the only part of Davison ' s initial year " not " filmed in 1981 was " Time - Flight " , the only other DVD release to bear the claim of an era lasting from 1981 - 1984 . All told , Davison ' s time in front of the cameras as the Doctor lasted from 9 January 1981 to 12 January 1984 — almost precisely the three - year tenure he had been advised by Patrick Troughton to undertake . Contents : Notes : It was released as issue 108 of " Doctor Who DVD Files " . VHS releases . This story was first released in VHS episodic format in the UK March 1994 , April 1994 in Australian and September 1995 in the US , with a cover design by Pete Wallbank .