On The Bridge
The final segment of The Key to Time is traced to the planet Atrios which The Doctor, Romana and K9 are soon to discover is engaged in a long interplanetary war with its neighbouring planet Zeos.
When the TARDIS arrive on Atrios, as the latest bombing raid by the Zeons comes to an end, The Doctor, Romana and K9 discover that the Marshal of Atrios, who is desperately seeking a means by which to win, intends to deploy a final strike to destroy the Zeons.
As they go in search of the sixth and final segment of The Key to Time, K9 is lured into a metal recycling system and The Doctor is forced to go in after him towards the furnace. After K9 is rescued they discover that the Marshal is being controlled by someone else, and receives his orders from a skull-like communicator behind a two-way mirror.
Intrigued The Doctor, Romana and K9 make there way to Zeos but discover the planet is deserted. Instead they find that the war between the two planets is being controlled from Zeos by a giant computer commandant called Mentalis. Using K9, as an interpreter, The Doctor learns from Mentalis that the computer was built by Drax, a Time Lord and old college friend of The Doctor’s. They also learn that Mentalis is planning a final strike to counter the Marshal’s similar plan even though this will result in everything being destroyed. The Doctor, with the assistance of Romana and K9, disables the computer to prevent both worlds from being destroyed.
The Doctor also learnt from Mentalis that there is third planet orbiting between Atrios and Zeos and that this is their true opponent. Called the Planet of Evil it is an invisible planet ruled by The Shadow, who, with his servants the Mutes, is working for The Black Guardian to try and gain control of the final segment before The Doctor can find it. The Shadow has captured Princess Astra of Atrios and is holding her under threat of torture in the belief that she can tell him where the segment is, but she does not know.
|The Marshal and The Doctor
To stop the Marshal, now on his way in a spaceship to deliver a final missile strike on Zeos, The Doctor builds a fake final segment for The Key to Time out of chronodyne, and in this form The Key to Time has enough power to create a temporary time loop, trapping the Marshal and his spaceship.
On the Planet of Evil, The Doctor encounters Drax, who it seems has been employed under duress by The Shadow. However, he agrees to help The Doctor. With K9 now under his control, The Shadow forces The Doctor to retrieve the five segments of the The Key to Time from the TARDIS. On exiting the TARDIS The Doctor is suddenly shrunk to a tiny size when Drax uses the dimensional stabilizer from his own TARDIS. Drax then uses the dimensional stabilizer on himself.
The Mute, who escorted The Doctor, returns to The Shadow with The Key to Time, and it is then that The Doctor realises that Princess Astra is herself the sixth segment. The Doctor though is powerless to stop The Shadow converting her into the crystal.
|Princess Astra and Romana
Using their diminished size, The Doctor and Drax smuggle themselves into The Shadow’s lair inside of the now-restored K9. Drax again uses the dimensional stabilizer, this time to return The Doctor and himself to their normal size. The Doctor is able to snatch all the segments that make up Key to Time and return, with Romana and K9, to the safety of the TARDIS.
After dealing with the Mentalis self-destruct (with the help of Drax) as well as the Marshal’s missiles (using the TARDIS to deflect them onto the Planet of Evil) The Doctor is finally able to complete The Key to Time.
This results in The White Guardian appearing on the TARDIS scanner who congratulates The Doctor on a job well done. He is now to hand over The Key to Time. But The Doctor, realises that this is a deception - due to The White Guardian’s lack of compassion for the fate of Princess Astra - and also the realisation that The Key to Time is far too powerful for any one being to control. He therefore orders it to re-disperse itself, much to the chagrin of The White Guardian who in his rage reverts to his true colour - Black.
Enraged The Black Guardian vows that The Doctor will die for his defiance. Princess Astra appears back on Atrios with her lover, Merak. The Doctor then fits a randomiser to the TARDIS controls, which means that in future no one, including The Black Guardian, will know where or when the TARDIS is going to materialise.