The Armageddon Factor

The Doctor, Romana and K9 conclude their search for the six disguised segments that make up the powerful Key of Time.

Arriving on the planet Atrios, the Doctor and his companions become embroiled in the Atrians’ seemingly endless war with their neighboring planet of Zeos.  Their ruler, the Princess Astra, has been kidnapped, leaving the deranged Marshal in sole charge of the war effort.  However, the Marshal is being controlled, receiving orders from behind a strange black mirror in the command center. 

The Doctor learns that no-one of Atrios has ever seen a Zeon, and travels to Zeos in an attempt to bring the war to an end, and hopefully find Princess Astra-who seems to be linked in some way to the Sixth Segment.  He finds the planet deserted, run entirely by a huge computer, Mentalis.  When the Marshal launches an all-out final attack on Zeos, Mentalis responds by activating its Armageddon Program-rather than lose the war, the computer will destroy everything.

However, a third planet hangs in space between Atrios and Zeos, shielded from their sensors.  This is the lair of the Shadow.  He has been waiting for the Doctor to arrive, manipulating the war to his advantage.  He is willing to use the Doctor’s own friends against him.  He wants the Key to Time.  Although the Doctor’s quest is nearly over, he begins to realize that he may lose everything.   For the Shadow is a servant of the Black Guardian…

• The final story in the Key to Time arc, "The Armageddon Factor" also is Mary Tamm's last appearance as Romana. 

• Shortly after filming she decided that her character had descended into the screaming companion role and decided not to return.  Her replacement was handled in the first story of the next season, "Destiny of the Daleks."  To make matters a little more confusing, Tamm's replacement was Lala Ward (who played Princess Astra in "Armageddon Factor").

• John Leeson (the voice of K-9) would also depart after the filming of this story.  David Brierly would replace Leeson for the 17th season.  Leeson would be absent for one year before his return for season 18, where his character was finally written out for good in "Warriors' Gate."

• The Doctor's "name" was revealed in this story by a classmate of the Doctor's from Gallifrey as Theta Sigma.  No other reference was made of this name until 9 years later in the Sylvester McCoy story "The Happiness Patrol" where the Doctor clarified that Theta Sigma was his nickname at school.  

• Veteran actor Valentine Dyall would appear as the Black Guardian near the end of this story.   Some years later he would reprise his role in a 3 story arc during the 20th anniversary season of Doctor Who ("Mawdryn Undead," "Terminus," and "Enlightenment").

• This story is also available on DVD as an individual title or as part of the Key To Time Boxset.

The Key To Time Arc
"The Ribos Operation"
"The Pirate Planet"
"The Stones of Blood"
"The Androids of Tara"
"The Power of Kroll"
"The Armageddon Factor"