Code Silver (audio story)

 was the third story in the audio anthology, Cyber-Reality, produced by Big Finish Productions. It was written by Guy Adams and featured Jemma Redgrave as Kate Stewart and Ingrid Oliver as Petronella Osgood.

Publisher's summary
The Cybermen are coming. Across the barriers between universes, conquering all before them. Conquering and converting.

As her troops fight a relentless enemy, Kate battles to lock down the threat. But how long can UNIT resist?

Plot
Inside the laboratory on Fortress Island, Josh Carter observes that several figures are marching through the dimension gate towards him and Kate Stewart. The two fire their guns at the figures to stop them, to no avail; several Cybermen emerge from the dimension gate, with one proclaiming that Kate and Josh's weapons are "insufficient". The partially-converted Shindi, Sam Bishop and Osgood state that Kate and Josh will be upgraded. Kate and Josh quickly exit the laboratory, before Kate broadcasts a Code Silver alert across Fortress Island. The duo then head to Kate's office, despite Josh's reluctance to not wait for reinforcements.

Back inside the laboratory, a Cyber-Controller exits the dimension gate before ordering Osgood to stabilise the dimension bridge and activate a signalling shield. Osgood does both, before stating the names and professions of herself, Shindi and Sam to the Cyber-Controller. Realising that Osgood's scientific knowledge will be useful, the Cyber-Controller forces Osgood to undergo a cranial upgrade, much to Osgood's discomfort. Now connected to the Cyber-Mainframe, Osgood requests that Shindi and Sam be left only virtually-converted; she has a plan to improve the Cybermen's dimension seeding protocols, but will need their assistance. The Cyber-Controller accepts Osgood's proposition, before ordering its subordinate Cybermen to take control of the fort, with the assistance of several mobile conversion units

Several UNIT soldiers engage the Cybermen in battle, meanwhile, Kate and Josh reach Kate's office, observing the battle via the security cameras. Kate discovers that the Cybermen are blocking communications from the fort, preventing her from calling for backup. Josh notes that the conversions of Osgood, Sam and Shindi are similar to the conversions of the lifeforms he encountered with Osgood and Shindi on the parallel Earth they visited. Kate hopes that their conversions can be reversed, and theorises that as the virtual reality headsets were broadcasting carrier waves, they must be able to transmit matter. She then quickly locks down the entire fort, stating that their mission was never evacuation; UNIT must stop the Cybermen spreading across Earth with whatever means necessary.

Returning to the security cameras, Kate and Josh observe several mobile conversion units; one "swallows" a soldier, and seconds later, a Cyberman exits the conversion unit. To gain a better understanding, Kate and Josh analyse the current situation; Osgood, Sam and Shindi are under mind control with limited physical conversion, potentially allowing for their conditioning to be reversed. The VR headsets took Josh, Osgood and Shindi to a parallel Earth under complete Cyber-control, despite the absence of any Cybermen. The humans stored in the pods underneath Piccadilly Circus were awaiting full cyber-conversion, and the Machine stated that "the flesh was useful" and gave "energy". Kate questions if the Machine was referring to electricity, theorising from such that these Cybermen's main invasion tactic is to upgrade worlds and transform them into power stations. The "dimension seeding" that the Machine referred to is the Cybermen's use of VR headsets to form bridgeheads into parallel universes. Josh questions why the VR headsets' control signal failed to work on him; Kate concludes that, as the signal was calibrated to human biology, Josh's Auton skeleton was enough to scramble it. Commenting that theorising under pressure is the "story of her life", Kate tells Josh to follow her to the communications room.

Back in the laboratory, Osgood analyses the dimension seeding protocols, commenting that it is simple but slow. The key to her strategy is the dimension bridge technology. Although the process of upgrading via the headsets takes extensive time to complete if there is a dimensional gap between the user and the signal, the process can be instantaneous if the mind of the user and the signal are in the same dimension. Osgood's plan involves broadcasting this signal worldwide, but the Cyber-Controller believes that no delivery system on this Earth would allow for the signal to be broadcast whilst also engaging the receiver's cerebral functions; Osgood then presents such a delivery system - a mobile phone. By reworking the Auctioneers' virtual-reality game, which itself is based on dimension bridge technology, Osgood can incorporate the mind control signal into a free, automatically-downloading smartphone app, allowing for the virtual conversions of millions of people across the planet.

To be continued

Cast

 * Kate Stewart - Jemma Redgrave
 * Osgood - Ingrid Oliver
 * Josh Carter - James Joyce
 * Lt Sam Bishop - Warren Brown
 * Colonel Shindi - Ramon Tikaram
 * Cyber-Controller / Cyber-Commander / Cybermen - Nicholas Briggs
 * UNIT Captain / Soldier - Katherine Jakeways

Continuity

 * Osgood tells the Cyber-Controller that UNIT's Black Archive would dispute claims of the Cybermen being unstoppable. (TV: The Invasion et al.)
 * Kate suggests that Josh's Auton physiology made him immune to the VR headsets' control signal. (AUDIO: Bridgehead, Armageddon)
 * Kate is aware of at least two separate origin points for the Cybermen: one in N-Space (TV: The Tenth Planet et al.) and one in a parallel universe (TV: Rise of the Cybermen / The Age of Steel) which attempted a invasion of N-Space. (TV: Army of Ghosts, Doomsday)
 * Kate speaks of the parallel evolution of the various Cyberman races. (TV: The Doctor Falls)
 * Osgood recalls the app created by the Auctioneers. (AUDIO: Game Theory)
 * Kate notes that UNIT has a stock of gold bullets for use against Cybermen. (TV: Battlefield)
 * Kate mentions that depending on the variant of Cybermen they are facing they are susceptible to radiation (TV: The Tenth Planet), Acetone (TV: The Moonbase) gold, (TV: Revenge of the Cybermen, Silver Nemesis) and emotion. (TV: The Invasion)