Traitors' Gate (audio story)

 was the second story in the audio anthology Masters of Time, produced by Big Finish Productions. It was written by Sarah Grochala and featured Jemma Redgrave as Kate Stewart, Ingrid Oliver as Petronella Osgood, James Joyce as Josh Carter and Chris Lew Kum Hoi as Jimmy Tan.

Publisher's summary
Osgood is a prisoner in the Tower of London, which is now a Vulpreen base. To rescue her friend, Kate must devise a plan to raid her former headquarters.

Inside the Tower, the Vulpreen are determined to access the Black Archive and the hugely dangerous alien technology it contains. How long can Osgood keep them out?

Plot
Osgood repeatedly lets Commander Sorgan into the Black Archive to find a particle isolator to make the Super Arches usable only by the Vulpreen before wiping her memory and doing it again, which Varliss puts a stop to after ten days. Sorgan puts Jimmy in a time bubble and threatens to age him to death or return him to the point of his birth, forcing Osgood into letting her into the Black Archive without using the memory filters. Osgood deceives her by pointing out a temporal rejuvenator, a device similar in appearance.

Kate, Josh and teacher Lorraine take a submarine down the River Thames and into Swordfish One's docking bay beneath the Tower of London. They split up to find a way out of the tunnels, which have been closed off with frozen time, and Lorraine is pulled through a Vulpreen Arch. Kate and Josh run into Ashley, the Tower's Ravenmaster, who explains how he was moved by a time glitch and sends a raven of death to make contact with Osgood.

When Varliss realises that Sorgan has again been deceived by Osgood, he recounts how he watched the death of Sorgan's brother and sends her to find the real particle isolator. Osgood allows her to have it once she threatens Jimmy, but she now intends to kill Varliss with a Sontaran fragmentation grenade and locks Osgood up with Lorraine in a cell which is soon visited by the raven. Osgood informs Kate of Sorgan's plan to assassinate Varliss by integrating the grenade with a Super Arch and gives the raven a time knife which can cut through frozen time, but the raven is shot at by the Vulpreen and only gets back to Ashley thanks to a time glitch.

Varliss pushes Sorgan through the Arch to kill her, having been warned of her deception, and Osgood deduces that Lorraine must be spying on Kate for Missy. Using the time knife, Josh frees Jimmy and Kate frees Lorraine, who locks her up before running into Osgood and confirming her suspicions; she has been hypnotised by Missy. Osgood, Josh and Jimmy chase Lorraine whilst Kate, having escaped her cell, confronts Varliss with her gun. Varliss captures her in a time bubble which she cuts through with the time knife, after which he flees using an Arch field.

Kate, Osgood, Josh and Jimmy are reunited and they hurry to the Arch in the tunnels to stop Lorraine from returning to Missy, but they find that she has already gone through and that the Arch remains activated. Kate sends Ashley back to base and heads through the Arch with Osgood, Josh and Jimmy to save Lorraine and confront Missy.

Cast

 * Kate Stewart - Jemma Redgrave
 * Osgood - Ingrid Oliver
 * Captain Josh Carter - James Joyce
 * Sorgan - Harriet Kershaw
 * Ashley Bassett - Stewart Scudamore
 * Lieutenant Jimmy Tan - Chris Lew Kum Hoi
 * Varliss - Alisdair Simpson
 * Lorraine Forrester - Donna Berlin
 * Vulpreen - Chris Naylor

Worldbuilding

 * Yeoman-Warder Ashley Bassett was the Tower's Ravenmaster.
 * Matilda is one of the ravens of death.
 * Sorgan's brother was Morgis.

Continuity

 * Osgood and Jimmy have been taken to the Tower of London by Varliss. (AUDIO: One Way or Another)
 * Josh mentions Swordfish One. (AUDIO: The Battle of the Tower)
 * The Tenth Doctor blew up a base beneath the Thames Flood Barrier. (TV: The Runaway Bride)
 * Osgood has been a captive of the Vulpreen for six months. (AUDIO: Time of the Vulpreen)
 * Sorgan mentions how Jimmy was one of the intruders on the asteroid ship. (AUDIO: The Vulpreen Encounter)