The Castle of Kurnos 5 (audio story)

 was the third story in the audio anthology Hearts of Darkness, produced by Big Finish Productions. It was written by David Llewellyn and featured Derek Jacobi as and Paul McGann as the Eighth Doctor.

Plot
"The Master" says that when he speaks he does not hear his own voice, and when he looks in the mirror he does not see his own face - he is the Doctor, and something has gone wrong. The story then flashes back to before the first episode, where the Master is conducting experiments looking for some sort of DNA match. After a while, he finds Meri - a girl from the nearby village.

The Doctor arrives by TARDIS in the village and finds Kilda, Meri's mother, looking for her missing daughter. Upon noticing him, Kilda has him arrested. She questions him and the Doctor swears that he's only just arrived, though he has an idea who might have taken her daughter. He tells her that he came here looking for the Master - he followed the Artron trail given off by his TARDIS - because the body of Cardinal Magos has been stolen from its tomb. Though he doesn't yet know what the Master wants with it, he knows that he used to idolise him as a child for being a brilliant and powerful Time Lord scientist. Eventually Kilda decides to trust him, and they agree to work together to track down the Master and rescue Meri.

The Doctor tracks his TARDIS' Artron trail and finds the ship beneath a layer of dried clay. When he tries to carefully investigate, he falls through the clay into the ship below, but is uninjured. He finds a map of the village and its surrounding area, and deduces that the Master has deliberately hidden his ship and plotted a secretive way to an abandoned castle because he didn't want to be spotted by any of the locals, though he's not yet sure why he requires so much secrecy.

Kilda and the Doctor leave on horseback, noting the stares of the other villagers. None of them think that he is trustworthy, but they believe Kilda to be a good judge of character so they don't interfere. When they come across vultures, the Doctor is distressed by Kilda's desire to feed the horses to them as a distraction, and instead scares them off with a high pitched noise from his sonic. Once they're safe, she suggests scaling the castle wall, but he realises there is a large drop on the other side so they wade through the ravine filled with old food waste instead, and crawl their way into the castle's kitchen. The pair are quickly attacked by centuries-old automatrons that functioned as the castle's defence system back when Magos lived here - he had ruled the area from his castle after being banished from Gallifrey for his experiments. The Doctor defeats the automatrons by sitting completely still, so the only movement for the two robots to track is each other/ They destroy each other, and the pair continue their search.

Back in the laboratory, the Master is pleased to find that Meri is the DNA match that he was looking for - she's a descendant of Cardinal Magos. This means that he will be able to use her body as a temporary host for the consciousness of Magos, as he has built a machine that can pull him forwards in time from just before his death and place it inside Meri's head. The Master speaks to him briefly, and learns the location of a device he had built and hidden, which he had named the Cognition Shift.

Once they find the Master and Meri, the Doctor shuts down the power to the castle as Kilda's species has better vision in the dark than they do. She grabs Meri and they flee to safety, leaving the two Time Lords alone.

To the Doctor's surprise, the Master had been expecting him. He'd even led him there, making the deliberate choice not to conceal his TARDIS' Artron trail, to leave the ship in plain sight, and to leave out the map to the castle. His old friend arriving had all been part of the plan - the Master needs a disguise when he steals the Cognition Shift, as it is in Dalek territory and being monitored by the Time Lords. He uses his machine to swap their consciousnesses, so they are each in the other's body.

Sensing a disturbance, Celestial Intervention Agency operative Narvin comes to capture the Master, and finds only the Doctor in the Master's body. He eventually believes that they have swapped appearances and sends him on a mission to find, stop, and potentially kill the Master.

Cast

 * The Master / The Doctor - Derek Jacobi
 * The Doctor - Paul McGann
 * Dorada - Sandra Huggett
 * Kilda - Tanya Moodie
 * Meri - Amanda Shodeko
 * Dieter - Henry Nott
 * Villager - George Fletcher
 * Narvin - Seán Carlsen

Worldbuilding

 * Cardinal Magos was banished by the High Council.
 * The Master mentions Rassilon.
 * The Time Lords have dreadnoughts surrounding the Lehar system.

Continuity

 * The Doctor speaks horse, something he would also later do in his Tenth (TV: The Girl in the Fireplace) and Eleventh incarnation. (TV: A Town Called Mercy)
 * The Doctor mentions Shada. (TV: Shada, AUDIO: Shada)