The Entropy Plague (audio story)

 was the 197th story in Big Finish Productions' monthly range. It concluded a trilogy of the Fifth Doctor's return to E-Space that began in Mistfall. It marked the death of Nyssa, after she sacrifices herself to allow her companions to return to N-Space.

Publisher's summary
A Great Darkness is spreading over E-Space. Entropy increases. In search of a last exit to anywhere, the TARDIS arrives on the power-less planet of Apollyon, where the scientist Pallister guards the only way out — a mysterious portal. But the portal needs power to open, and the only power Pallister can draw on is the energy contained within the molecular bonds of all living tissue...

The Doctor, Nyssa, Tegan and Turlough soon learn that neither Pallister nor his ally, the space pirate Captain Branarack, will stop at murder to ensure their escape. But they're not the only menace on Apollyon. The Sandmen are coming — creatures that live on the life force; that live on death.

Death is the only way out into N-Space. Death, or sacrifice.

But whose death?

Whose sacrifice?

Plot
to be added

Cast

 * The Doctor - Peter Davison
 * Turlough - Mark Strickson
 * Nyssa - Sarah Sutton
 * Tegan - Janet Fielding
 * Cherryanne - Catherine Skinner
 * Pallister - Robert Duncan
 * Branarack - John Voce
 * Robot - Alastair Mackenzie

Uncredited

 * Adric Traken - Alastair Mackenzie

Continuity

 * The Doctor tells Adric Traken that the TARDIS accidentally travelled through a CVE to E-Space. (AUDIO: Mistfall)
 * Adric Traken refers to his encounter with his mother Nyssa on Valderon one month earlier. (AUDIO: Prisoners of Fate)
 * The Doctor refers to the fact that Tegan was kidnapped from Isenfel. (AUDIO: Equilibrium)
 * Nyssa refers to the destruction of Traken by in 1981. (TV: Logopolis)
 * The Doctor states that he is not entirely sure of Turlough's species. (TV: Planet of Fire)
 * The Doctor refers to the fact that he previously saw the Gateway during his fourth incarnation. (TV: Warriors' Gate)
 * During her farewell to the Doctor, Nyssa says "I'm needed; you know that, Doctor." At an earlier point in her personal timeline but a later point in his, she told him that the same was true of him. (AUDIO: Winter, TV: The Caves of Androzani) 
 * Nyssa refers to their visit to London during the outbreak of the Bubonic plague in September 1666. (TV: The Visitation)
 * The stars in E-Space go out as the universe dies. The same thing once happened to N-Space in an alternative timeline in which the Daleks were able to detonate the Reality Bomb. (TV: Turn Left)
 * Tegan refers to her encounter with Alzarian spiders on Alzarius. (AUDIO: Mistfall)
 * Nyssa recalls the death of the Monitor on Logopolis. (TV: Logopolis)
 * Tegan once again says, "Rabbits!" (TV: Logopolis, Frontios; AUDIO: Psychodrome, The Emerald Tiger)
 * Turlough recalls their reunion with Nyssa on Helheim in 3530. (AUDIO: Cobwebs)
 * Nyssa's contact with the Sandmen reverses the rejuvenation that she experienced in India on 31 December 1926. (AUDIO: The Emerald Tiger)
 * Nyssa refers to the Groves on Traken. (TV: The Keeper of Traken)
 * The Doctor says he keeps a crank handle for when the TARDIS loses power. (TV: Death to the Daleks)