The Doctor and Romana have five pieces of the Key of Time and are beginning their search for the sixth. The Doctor tries to park the Tardis in orbit around Atrios to see where their final mission will take them. Initially the Tardis has trouble finding Atrios but soon discovers it, though millions of miles where it should be. The Doctor realizes this will not be the typical mission, more may be involved than meets the eye.
The planet Atrios is at war with Zeos and on the brink of loosing the war. Princess Astra and medic Merak are romantically involved, but more importantly secretly trying to contact Zeos in hopes of ending the war. All attempts by Astra have failed and she's worried the Marshall is suspicious. Merak assures her they are still safe, or the Marshall would have done something about them trying to contact Zeos. Shortly after, the Marshall gives orders Princess Astra is to be locked away in a part of the city teaming with radiation.
The Marshall is determined to make sure he wins the war at all costs. Several times he 'meditates' in front of a mirror, much to the puzzlement of his right hand man Shapp. Shapp notices he seems to be talking to someone. Once the Doctor arrives, the Marshall blames him for the death of a guard, but when the Doctor claims to be able to help win the war, the Marshall takes a liking to the Doctor and is willing to work with him.
The Doctor tells the Marshall he'll need K-9 to help build a shield to stop the Zeons from attacking, but Shapp informs them K-9 was found and sent the furnaces to be melted down (per the Marshall's order). The Doctor runs after K-9 in hopes of saving him from being melted into slag. While doing so, Romana notices the Marshall is very agitated and mutters to himself, "the Doctor does not die.... yet."
The Doctor quickly discovers the Marshall is actually a puppet to some unknown shadow. He discovers a trans-mat which takes them, not to Zeos, but to a hidden base between both planets. On this planet is Mentalis, a computer that has been waging the war the Marshall has been fighting. K-9 talks to Mentalis and informs the Doctor if the Marshall is successful in his attack, Mentalis' fail-safe is to blow itself up, taking Atrios and Zeos with it.
While trying to figure out who this mystery shadow person is the Doctor runs into Drax, another Timelord. Drax recognizes the Doctor but calls him Theet. The Doctor is horrified to find out Drax built Mentalis, though under pain of death by the Shadow. It doesn't take long before the Doctor convinces him to switch sides and help the Doctor. Drax agrees Timelords should stick together and agrees to help. He starts off building a gun which can shrink people.
Once the gun is complete, Drax uses the gun on the Doctor and himself. The Doctor actually wanted Drax to shrink one of the Shadow's guards. Drax hadn't realized the Doctor opened his Tardis, which the gave the guard access. He went in and retrieved the first five segments of the Key to Time, claiming to know where the sixth piece was. The Doctor uses his miniature size to infiltrate the Shadow's domain (who turns out to be a servant of the Black Guardian). He reveals that Princess Astra is actually the final segment to the Key to Time. The Doctor grabs the completed Key and runs for the Tardis with Drax, Romana and K-9.
Safely in the Tardis and with Drax happily departed, the Doctor is contacted by the White Guardian who immediately tells the Doctor to release the Key to Time to him. When the Doctor asks what will happen to Astra, the White Guardian tells him she is a sacrifice for maintaining peace in the galaxy. This callous view of life makes the Doctor realize the White Guardian is really the Black Guardian in disguise. Not fooled, the Doctor disperses the Key to Time, the segments returning to their original forms, including Princess Astra. Now reunited with her lover Merak.
WHO: Romana I
DEPARTURE LOCATION: In the Tardis (Time and Location unknown)
REASON FOR DEPARTURE: It's unclear if Romana regenerates after being tortured by the Shadow, or simply wanted to change her appearance. Either way, it's not talked about on-screen and doesn't happen till the next episode.
NEXT: Return To Skaro