On the Moon, the Doctor is looking for his TARDIS in Hangar 7 when he is confronted by the giant robot known as the Agent .It addresses him as ‘Brother Varlos’ and tells him he has been sentenced to death…
The Doctor tells the robot that there has been a mistake and asks why it thinks he is Varlos. The Agent tells him that only Varlos knew the whereabouts of the Eternity Crystal. The Doctor says he doesn’t know what the Crystal is so the Agent informs him that the Crystal was considered too dangerous after the Mordane incident and has to be returned to the robot’s masters. The Doctor makes the robot scan him to prove that he is not Varlos. As the Agent does so, slipping into a trance-like state, the Doctor escapes. He locates the TARDIS on board a freighter due to fly to Earth. He is just about to step aboard the freighter when the Agent catches up with him.
On Mordane, three humans walk through the vast graveyards. As night falls they erect an Intelligent Plastic survival Pod and shelter inside. Catz is a young girl with a secret mission. She is accompanied by Captain Gomez and Jitan Chandra. Their ship has made an emergency landing on Mordane after a mysterious systems failure and the trio has been crossing the graveyards in search of help. By day the graves are silent but at night the dead awake and assault their tent with ferocity. While her companions sleep she checks that she is still on course to her goal by consulting a map on her laptop.
The Agent confirms that the Doctor is not Brother Varlos but thinks he knows where the Crystal is; in fact it is already inside the TARDIS within a stasis cabinet. The Doctor pretends that the TARDIS is merely a box where he stores things and says he will look inside for the Crystal.
After the night is over, Mordane returns to its peaceful state. The three humans continue in their search for metals to make repairs to their ship. Catz says she has scanned the area and that there are signs that what they want can be found in the catacombs.
The Doctor steps into the TARDIS and sets the co-ordinates for Mordane. As he dematerializes the Agent leaps onto the TARDIS and clings tightly as they travel through space at impossible speeds. His grip is loosened as they enter Mordane’s atmosphere and he plunges to the surface, teleporting to a safe spot at the last moment but not without the risk of being destroyed on impact. Inside the TARDIS the Doctor has noticed that something has made the trip somewhat erratic but has no idea what it was. He materialises the TARDIS in the graveyards that cover the planet. He steps out to find three humans pointing guns at him.
The Doctor quickly surmises from their dirty clothes and their conversation that these three humans have recently crash landed on the planet and offers to help them fix their ship. He introduces himself and asks what happened to their craft. Chandra tells him he suspects a burnt out thrust regulator. Catz says they just need some trisilicate to make the repair. They tell him that have to locate it quickly before the night falls and the dead walk again.
They arrive at the doorway to a memorial as night suddenly falls. Corpses begin to shuffle towards them from every direction and they run through the door into the catacombs. They shut the door behind them. Catz says that the catacombs are mostly empty. The Doctor thinks back to the animated dust on the moon and considers this too similar to the re-animated corpses to be a coincidence. A group of skeletons attack them but they manage to fight them off and the Doctor and Catz escape down a tunnel. Gomez and Chandra flee down another tunnel, firing energy bolts at the walking dead with little effect.
In the tunnels Catz is using a flaming torch to guide their way. She tries to consult her map without the Doctor seeing but his Time Lord eyes are too sensitive for her to get away with it. He doesn’t say anything but follows her into a massive circular cavern. She confidently leads him down one of several passages that extend away from the cavern. Suddenly, the Agent steps into the passage and picks up Catz. It demands the Eternity Crystal from the Doctor or Catz will die.
Elsewhere in the catacombs, Chandra tells Gomez that he knows where Catz was heading. He r grandfather’s grave is nearby. Gomez speculates that the enforced landing was part of a plan and that Chandra is implicated. They are interrupted by the arrival of dozens of zombies that have broken in from the planet’s surface.
The Doctor tells the Agent that the Crystal is back in the TARDIS. He is impressed when the Agent says that it has scanned the TARDIS and found that it has extra dimensions; not many technologies can penetrate the outer shell. The Doctor suggests that they swap information so that he can Help the Agent. The robot agrees and tells him that the Crystal was the centerpiece of a machine designed to create life. When the Doctor hears the Darksmiths mentioned he tells Catz that they are artisans, able to fabricate new realities from the raw matter of the universe. The Agent says he was created by the Darksmiths to track down the Crystal which was meant to be used on Mordane by Varlos as part of the life-creating machine. However, Varlos took the Crystal centuries ago, leaving the machine unstable. Since then the Darksmiths have been waiting for a sign that the Crystal was still in existence. His explanation is cut short by dozens of zombies entering the passage.
The Doctor says that the Agent cannot enter the TARDIS without him so they agree to help each other. The Agent holds the walking dead off with an energy ray while Catz leads the Doctor to her grandfather’s tomb. They enter a magnificent chamber. Catz tells the Doctor that her grandfather was King Morrish a’Jethwa of the Folflower Royal Family. She looks on top of the sarcophagus but is disappointed to find that her ‘inheritance’ is missing. She immediately suspects Chandra because he was the only person she trusted with her secret.
Meanwhile, deep in the tunnels, Chandra is holding a golden torc as he and Gomez return to the exit.
Catz tells the Doctor her history. Her grandfather was the ruler of five planets in the Folflower system. His death triggered a move for democratically elected leaders and her father was deposed in a bloodless coup before his coronation. The royal family was sent into exile and Catz was brought up on tales of the fairness and might of her ancestors as they ruled the system. As the republic that replaced them founders in greed and power struggles the people have been calling for a return of the royal family. Since her father is too old to wield the power Catz has taken it on herself to find the torc, the symbol of her right to rule, and return home. The Doctor asks why they have waited so long to do this and Catz tells him that Mordane has been a lost world. It was quarantined eighty years earlier and then records of its whereabouts erased. Her father spent his life looking for the planet and passed on his data to Catz. She has only recently been able to locate Mordane and her grandfather’s tomb.
The Doctor says they need to get back to her ship before the others leave without her but she shows him the trisilicate link from the engine that Chandra helped her remove. They escape from a new group of zombies by finding a service tunnel to the surface. Unfortunately it is still night and there are thousands more corpses walking in the night air. Just as it seems there is no escape Captain Gomez flies in aboard a helicopter to rescue them.
On Karagula the Darksmiths are puzzled that the Crystal has not been returned and that the Agent is now on Mordane. Drakon is worried that Varlos has returned to the planet to complete his work and that the dead will rise on a million worlds and threaten the Universe.
The Doctor helps Gomez and Chandra to repair their ship and then find Catz looking at the torc. They discuss its ability to generate a force field that allowed Chandra and Gomez to walk unharmed back to their ship. Catz apologises to Chandra for doubting him and the crew offer to help the Doctor in his next task: turning off the signal that has re-animated the dead.
Chandra uses the helicopter to fly the Doctor and Catz to one of the planet’s administrative towers where the machine that Varlos built is located. They place the crystal into the machine. On Karagula its signal is immediately identified and the Darksmiths are confident that the Agent will soon have it in its possession.
Night falls and Chandra has to leave the tower in the helicopter as the dead close in. The Doctor finds that it is possible to reverse the machine’s signal but there is a distributor box overloading on a satellite dish outside. Catz uses the protective torc to walk through the dead to the box where she uses the screwdriver the Doctor lent her to make the necessary repairs. Immediately she has finished the zombies decay into dust. She returns to the Doctor who tells her that Mordane is at peace once more. Chandra returns to pick up Catz. After a brief farewell the Doctor walks back into the tower to retrieve the Crystal from the machine but when he gets there he finds that it has gone.
Drakon receives the news of the Agent’s return to Karagula with pleasure. Now that the Crystal is back in their hands the Darksmiths can resume their greatest work.
Meanwhile, the Doctor is determined to take the Crystal from them to protect the universe. He sets the controls of the TARDIS for Karagula. His mind is filled with dread at the thought, knowing that event the Time Lords never approached the planet. Hoping that his trip will not be one-way, he dematerializes his ship.