Once he miniaturizes the Doctor, Drax turns the dimensional stabilizer weapon on himself and shrinks down to the same size. The Doctor runs over to the wall and ducks into a crevasse, avoiding the mute, and explains to the confused Drax that he wanted the technician to shrink the mute. The TARDIS door is open and there’s no way to get to it, and they can’t enlarge themselves inside of the crevasse.
Inside the TARDIS, the artificial chronodyne segment is burning out and the time loop is stretching ever further. The Marshal gets ever closer to successfully firing the missiles.
On Atrios, Shapp and Merak are unable to get through to the Marshal. Merak wonders what Astra knows of the segment, and suspects that it may have affected her in a way that could show up on her medical records.
The Doctor ponders what the Shadow told him about the sixth segment and concludes that Astra has it. He and Drax go further into the crevasse.
The Shadow realizes that the Doctor has left the TARDIS door open and goes to get the first five segments. Once he leaves, Astra realizes that her destiny lies within the Shadow’s fortress. She insists on staying and explains that she’ the sixth princess of the sixth dynasty of the sixth Royal House of Atrios, and the time of her becoming is at hand.
The Doctor and Drax go to Drax’s workshop and decide to use K-9 to get close to the Shadow.
The Shadow is unable to enter the brightly lit interior of the TARDIS and sends one of his mutes to fetch the five segments.
Romana realizes that Astra is the sixth segment of the Key of Time, shapeshifted into a living being. On Atrios, Merak has reviewed Astra’s medical records and learned that there’s a genetic anomaly, passed from one generation to the next. Her every living cell is part of the Key of Time, and she must be destroyed to save them all.
The Doctor and Drax enter K-9 and send him back to the Shadow with orders to convince him that he’s still under the Shadow’s control.
Merak returns to Zeos and forces one of the mutes to take him to the Shadow’s planet. Upon arrival, Merak knocks out the mute and dons his robe. The Shadow enters his command chamber and Merak slips in with the other mutes. He tells Astra that her destiny has arrived. She touches the Tracer… and transforms into the sixth segment.
Shocked, Merak reveals himself, while K-9 blasts through the wall. The robot dog apologizes and warns the Shadow against Merak. He reports that the Doctor and Drax have been eliminated, and the Shadow has Romana and Merak stay to see his triumph. The Doctor and Drax emerge from inside K-9 and use the dimensional stabilizer to enlarge themselves to normal size. The Doctor grabs the assembled five segments and the sixth piece and uses the light to repel the Shadow. Drax stays to hold the Shadow and his mutes back while the Doctor, Romana, K-9, and Merak go to the TARDIS.
At the TARDIS, Merak goes back to look for Astra. The Doctor seals the TARDIS and Romana notes that they’re murderers, pawns sent to the do the Guardian’s dirty work. The Doctor isn’t happy at the situation either, much to Romana’s disgust. The Doctor tells her to set the coordinates for Zeos so that they can stop the Marshal. The time loop has stretched to its ten-second limit. The TARDIS arrives on Zeos and Drax arrives via the transmat with Merak and K-9 in tow. He checks his notes and determines what wire the Doctor has to cut. They stop Mentalis’ countdown and remember that they have to deal with the Marshal.
The Marshal finally fires the missiles, only to watch in horror as they’re redirected… to the Shadow’s planet. The Shadow is blasted apart, begging the Black Guardian for forgiveness. The Black Guardian informs his servant that his death is encompassed in his designs, and the Doctor will turn the key over to him.
In the TARDIS, the Doctor explains to Romana that he used a deflective force field to bounce the missiles to the Shadow’s planet. K-9 departs for a contract job on Atrios doing reconstruction with the Marshal. They drop Drax off and Romana prepares to set the coordinates for Gallifrey. The Doctor wonders if anyone can handle the responsibility of absolute power that the Key possesses.
The White Guardian appears on the viewing screen and offers his congratulations. He asks for the Doctor to turn over the Key of Time. The Doctor points out that the White Guardian initially explained that when assembled, the Key would stop the flow of time in the universe so that the natural forces could be rebalanced. The Doctor prepares to release it, and then points out that the Key is already assembled. The White Guardian says he’ll take the Key for safekeeping. The Doctor then asks about the sixth segment, and that it’s a human being. If they’re maintained, Astra is doomed. The White Guardian admits it’s regrettable but he has no choice with the fate of the universe at stake.
The Doctor prepares to turn the Key over… and then tells it to stay where it is and activates the TARDIS defense screens. The White Guardian transforms into the snarling visage of the Black Guardian. The Doctor explains that the White Guardian would never have such a callous disregard for human life. The Black Guardian is powerless as long as the Doctor holds the Key. On his signal, Romana dematerializes the TARDIS and the Doctor disperses the six segments.
On Atrios, Astra reverts back to her human form and is reunited with Merak.
On the TARDIS, the Doctor explains that he scattered the six segments again. When Romana wonders where they’re going, the Doctor explains that he has attached a randomizer to the TARDIS navigational controls. As long as he has no idea where he’s going, the Black Guardian can’t find them. The two Time Lords depart on the first of their randomly plotted adventures.