The Children of Seth (audio story)

 was the third story in the third series of The Lost Stories, produced by Big Finish Productions. It was adapted by Marc Platt, from the original script by Christopher Bailey, and featured Peter Davison as the Fifth Doctor, Sarah Sutton as Nyssa and Janet Fielding as Tegan Jovanka.

Publisher's summary
During one of Nyssa's experiments, the TARDIS' temporal scanner picks up a message: "Idra". Just one word, but enough to draw the Doctor to the Archipelago of Sirius.

There, the Autarch is about to announce a new crusade. A mighty war against Seth, Prince of the Dark...

But who is Seth? What is the secret of Queen Anahita, Mistress of the Poisons? And what terror awaits on Level 14?

Part one
Despite being forbidden from visiting her husband without his express permission, the disfigured Queen Anahita sneaks to the Autarch's rooms at night and has her servant, Mira, open his sonic lock. High Minister Byzan finds them and accuses them of attempting an assassination, sending Anahita back to her rooms and having Mira reassigned to Level 14 with her mind wiped after she fails to divulge any information. The next day, on the Autarch's birthday, Byzan prepares a speech declaring war on the planets outside of the rim on the pretence that they are sheltering their enemy, Seth.

Nyssa has devised a program based on probability theory and transmits a question from the TARDIS which is answered by a message from the Archipelago of Sirius saying "Idra". The Doctor attempts to materialise the TARDIS at the message's origin, but the ship is held in a time-space probability lattice and is unable to decide which of the possible futures to travel into. A low-level intelligence drone breaks in and scans the console, designating the Doctor, Nyssa and Tegan as hostile intruders and blowing up whilst attempting to calculate the TARDIS's infinite coordinates. The ship gets through the lattice and materialises in the Imperial City in the Sirian Empire.

The Doctor, Nyssa and Tegan are mistaken for off-world guests and listen to Byzan's speech. Shamur, who calls himself a wanderer between heaven and the world, arrives with weapons and a helmet from the Plains of Ragnarok in search of his granddaughter and says that he has come to speak with the Autarch, but Byzan orders him to leave. He hits Dario Varidi with his stick when he attempts to remove him by force and Nyssa draws attention to herself and the Doctor when she calls Dario an android. Realising that they are the ones who breached the defence net, Byzan gives them the choice of interrogation or a mind wipe.

Part two
Tegan sees the Doctor and Nyssa being arrested and is kept from getting involved by Anahita, who met the Doctor in one of his previous incarnations when she was a student and is the one who sent the message to the TARDIS, "Idra" being her original name. She covers for Tegan by claiming that she is her new servant, gifted to her by the Autarch, and takes her to the Imperial Judiciary to rescue the Doctor and Nyssa, imprisoned with Shamur. Byzan visits the three prisoners disguised as a priest and listens to Shamur's story of the battle on the Plains of Ragnarok being a hallucinogenic-fuelled exercise in which the imperial soldiers killed one another.

Byzan has Albis, his android private secretary, take Nyssa for interrogation, but Albis shows initiative and subjects her to a mind wipe and an assignment to Level 14. Tegan arrives in the dungeon and kisses the guard with a paralysing agent, allowing her to release the Doctor and learn what has happened to Nyssa before heading to Anahita's quarters where they are joined by Shamur, who has had a fruitless audience with the Autarch. A patrol drone like the one that broke into the TARDIS attacks Anahita, but the Doctor has it determine an answer to Nyssa's question and uses the opportunity to plug himself into the defence net. He is overwhelmed with numbers and loses his sight.

Part three
The Doctor, only able to see numbers, is guided back to the TARDIS by Tegan and Shamur and sends Tegan back to Anahita with a copy of her book, The Trick of Darkness, which she gave him when they last met. He directs Shamur to pilot the TARDIS to Level 14 and they search for Nyssa and Mira in the mists of hallucinogens keeping the memories of those condemned there suppressed.

The book is confiscated from Tegan and she is chased by a drone, finding Anahita and infiltrating Byzan's sanctum from which they can view the Doctor drawing in the sand on Level 14. The Autarch delivers the speech written for him by Byzan, declaring war against those supposedly harbouring Seth. To lead this war, the Autarch abdicates in favour of Byzan, after which the broadcast is interrupted with an image of the Doctor threatening the world and identifying himself as Seth.

Part four
The Doctor accesses Byzan's plans on the system, which he learns is called the Autonomous Logical Binary Intelligence System (or "Albis"), before he is closed off from it and his sight restored. He is able to see Nyssa, who believes that she is the Doctor as a result of the hallucinogens affecting her memory, and that Shamur has been killed by a now-inactive drone. He leaves Nyssa and the TARDIS on Level 14 and goes with Tegan to convince the former Autarch, Siris, to flee, but the city is attacked by androids controlled by Albis.

Level 14 is sealed off and the people to be culled, so Tegan uses Siris's address system to tell her to break out with a direct beam disruptor in Shamur's possession and lead the people out. Anahita is taken hostage by Byzan, who wants the Doctor to take him in the TARDIS to escape Albis, and reveals to Siris that she used the name "Seth" in her book to embody the threat that a civilisation needs; Byzan implemented her ideas without changing the name, not realising that his own creation was the true threat. The Doctor pretends to be Seth and has Nyssa, her memory restored, throw the disruptor to Byzan for him to use on Albis. Although reluctant, he does so and the androids are defeated.

Nyssa and Tegan have recently been considering leaving the TARDIS, but they have chosen to stay. The Doctor informs them that the worlds of the empire might be made independent and that Siris and Anahita will be retiring to a secluded colony whilst Byzan is put on trial. Unbeknownst to the travellers, Byzan has been poisoned after a visit from Anahita. Sirus asks Anahita to make sure that she has her bottles packed for their new home as they never know when her poisons will be needed.

Cast

 * The Doctor - Peter Davison
 * Tegan Jovanka - Janet Fielding
 * Nyssa - Sarah Sutton
 * Queen Anahita - Honor Blackman
 * Byzan - Adrian Lukis
 * Autarch Siris - David Warner
 * Shamur - Vernon Dobtcheff
 * Albis - Matt Addis
 * Mira - Emerald O'Hanrahan
 * Radulf Varidi - John Banks

Worldbuilding

 * Tegan makes tea for herself and Nyssa.
 * Timor was Mira's father. He died with honours on a campaign.
 * Dario Varidi is Radulf's son.
 * Tegan describes herself as "a fully paid up Aussie republican" and "downright Bolshie."
 * The Doctor previously visited Sirius during one of his first four incarnations.
 * Anahita wrote The Trick of Darkness.
 * Per the Doctor's instructions, Shamur uses the fast return switch to return the TARDIS to Sirius.

Continuity

 * The Doctor believes that the TARDIS team's recent visit to Luparis had a therapeutic effect on Nyssa (AUDIO: Hexagora) following her brainwashing at the hands of the Elite on Florana. (AUDIO: The Elite)
 * Tegan does not regret her decision to remain in the TARDIS as opposed to returning to Brisbane, Australia in 1983 with her former boyfriend Mike Bretherton. (AUDIO: Hexagora)
 * The Doctor would become blind again in his twelfth incarnation. (TV: Oxygen)