Doctor Who-The Armageddon Factor

                    The quest for the final segment of the Key To Time leads the Doctor,Romana and K-9 to the sister planets of Atrius and Zeos. Upon arrival it is discovered that the planet is in the middle of a long standing nuclear war. Princess Astra,the last surviving member of the royal family of Altrius and her love interest the doctor Merak are both pledging to Marshall,the head of the war department on Altrius to end the slaughter. Upon attempting to assist the seemingly violence crazed Marshall,the doctor discovers Marshall is himself under some type of control. Astra is than captured and held prisoner on an artificial Planet Of Evil. This planet is the domain of the malevolent Shadow,who is intent on getting his hands on the sixth segment of the key,even to the point of manipulating Astra to do his bidding. After nearly being hoodwinked through the brainwashed Astra the doctor,Romana and K-9 penetrate into this third planet to find the war has been masterminded by a supercomputer called Mentalis,programmed to create a stalemate whereby both Altrius and Zeos would destroy themselves. 

               He then creates an artificial sixth segment from chronodyne to use the key to create a time loop before Marshall launches a final assault on Zeos. In a literal race against time against the deteriorating loop,the doctor encounters a fellow time lord in his former classmate Drax,who was himself hoodwinked by the shadow into helping construct Mentalis and therefore starting off the war between the worlds. Drax,somewhat the survivalist agrees to help defeat the Shadow. Upon Astra and Romana being captured again,the doctor learns the Shadow is his equivalent for the Black Guardian,searching for the other five segments of the key for the opposite purpose as the doctor. It is at this point that Astra herself is,and was born to be by genetics,the sixth segment to the key. Before the time loop is broken the Doctor is able to escape to the TARDIS after which time the Marshall's weapons end up firing on the Planet Of Evil,not Zeos whose population have all been killed. Thrawting the Shadows mission,the Black Guardian disguises himself as his counterpart to convince the doctor to return the key to him. For this part,the doctor sees through his plan and redistributes the segments of the key back into space so at least they would not end up in evil hands.

           This is the most complex and densely written story in the Key To Time series. In it reality,matter,good and evil are bent in so many possible directions it's hard to see how everything comes into one singular focus. But somehow it all does. Although the doctor technically fails in his mission to reconstruct the Key To Time,he does in another way fulfill the White Guardians intention of restoring harmony by creating a stalemate: the black guardian isn't able to get the key for one reason. Neither the doctor or Romana are comfortable with Astra literally being genetically constructed to be the end source to universal harmony as the sixth segment. So by saving her life in addition to keeping the key out of the black guardians villainous hands,a balance (of sorts) was restored. As for Astra,Lalla Ward's stellar performance in the demanding role not only earned her the respect of the programs staff but of Tom Baker. So when Romana regenerated in the beginning of the following season of Doctor Who,Ward was chosen to replace Mary Tamm in that role. The idea of Romana taking on Astra's form was even worked into the plot of that story. Overall this is a fine conclusion to the Key To Time series with a very unexpected and morally fascinating conclusion.