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.
The Entropy Plague contains examples of:
- Be Careful What You Wish For: The Space Pirates sought to find and kidnap someone to power the portal, someone strong-willed and with "fighting spirit". They kidnap Tegan, who threatens to "put her fighting spirit right where it hurts".
- Bittersweet Ending: The TARDIS crew escape E-Space, which is also saved, but at the cost of Nyssa remaining trapped there without any hope of return, never able to see her children again.
- Book Ends: The first time we ever see Nyssa is as a young woman, tending the groves in her home on Traken. This is chronologically her final story, and she spends her elder years retiring from adventuring and medical care to look after her own gardens in E-Space.
- Call-Back:
- Adric Traken, Nyssa's son, recalls the last time he saw his mother, and her promise to come find him after a month.
- Nyssa brings up the last time they encountered entropic energy in "Logopolis", when it destroyed her home planet and vaporized the Monitor.
- The Doctor mentions the Gateway seen in "Warriors Gate".
- Catchphrase: Tegan's "Rabbits!" crops up again.
- Continuity Nod:
- To help them recover from their exposure to entropy, the Doctor has Tegan and Turlough spend several hours in D-Sleep.
- The crank handle the Doctor uses to open the TARDIS doors when it loses power comes in handy, though it is now located underneath the console rather than beside the doors.
- Continuity Snarl: Nyssa attempts to buy food at the Apollyon market with her hair comb, which Tegan notes is just about the last thing she had left from Traken, but this was visibly pulled from her hair and left behind all the way back in "Castrovalva", decades ago from Nyssa's perspective.
- Eyeless Face: The Sandmen's eyes disappear during their transformation, becoming hollow sockets instead.
- Foregone Conclusion: Since Nyssa never reappeared on the television series after "Terminus", her departure was inevitable.
- Get Out!: Adric says this to the Doctor, being understandably furious with him for not stopping Nyssa's self-sacrifice.
- Grand Finale: For the reunited team of the Doctor, Tegan, Turlough and the older Nyssa, which began in "Cobwebs", five real world years and fourteen stories ago.
- Heroic Sacrifice: Exactly who gets to make one and when they get to do it is the subject of much debate throughout the story. Ultimately, Nyssa gives up her youth and her only chance of returning to her family to power the portal that will get her friends home.
- Human Popsicle: Adric's sister Neeka has been woken up from her cryo-sleep and cured of Richter's Disease, though because she was frozen for so long she is now physically younger than Adric despite technically being his older sister. When talking to her on the phone, Adric jokes about her being the "baby of the family".
- I Choose to Stay: Nyssa, for the second time.
- In Medias Res: The story begins with the Doctor arriving at Adric Traken's home to tell him that his mother won't be coming back, before he (and Tegan and Turlough) go back to the beginning and explain.
- It's Personal: Nyssa is more emotionally affected by the entropic destruction on Apollyon than the others, since it directly recalls the way her home planet of Traken was destroyed.
- Layman's Terms: Thoroughly exhausted with the whole story towards the end, Tegan demands that the Doctor do his exposition of the latest inter-dimensional crisis in words of one syllable.
- Maybe Ever After: Even though Nyssa closes the CVE, saving her friends and preventing the immediate destruction of E-Space, it still only has a few centuries left before entropy consumes it again, not long at all in universal terms. The apparent birth of a new star near the end of Nyssa's life, however, could mean that there is a chance life could continue to flourish there.
- Mugged for Disguise: Cherryanne and Nyssa strip the wiring out of a sentry robot so that Cherryanne can disguise herself in it to infiltrate the prison and rescue Tegan.
- Narrator: All four members of the TARDIS crew take turns narrating parts of the story to Nyssa's son, Adric.
- Pity the Kidnapper: The Doctor comments that he's not sure whether to feel more sorry for Tegan or the pirates who abducted her.
- Plot-Relevant Age-Up: Nyssa's exposure to the entropic energy completely reverses the effects of her previous rejuvenation, making her appear even older than she seemed when the TARDIS team first re-met her on Helheim in "Cobwebs".
- Pragmatic Hero: Turlough in spades, and this is the first time that his different outlook clashes enough with the Doctor's to cause a real argument between them. While Nyssa and the Doctor argue about which of the two of them should sacrifice themselves to save everybody, Turlough wants to offer up one of the pirates instead, firmly believing that the good the Doctor and the TARDIS crew do for the universe makes them more deserving of living than the murderous, evil pirates. The Doctor is furious that Turlough could have spent so long travelling with him without learning to see things his way, while Turlough is frustrated that the Doctor's first solution is always to sacrifice his own life instead of trying to Take a Third Option.
- Senseless Sacrifice: Poor Cherryane straps herself into Pallister's machine and has her youth painfully drained from her, and it doesn't buy the others even a full minute of escape time.
- Shout-Out: Tegan derisively calls Captain Branarack "Long John Silver".
- Space Pirates: Captain Branarack and his crew are a solid Type 2, with plenty of "Arrgh!"s and "scurvy dog"s thrown about.
- The Stars Are Going Out: Happening all over E-Space as a result of entropy dramatically shortening the universe's lifespan. At the end, a new star seems to appear, so all might not be quite as dire as it appears.
- Teeth-Clenched Teamwork: Briefly between the Doctor and Captain Branarack. Tegan is particularly outraged, since Branarack had recently tried to sacrifice her.
- Trapped in Another World: Nyssa lives out her final years on the planet Apollyon in E-Space, having given up her only chance of getting home to allow her friends to do so.
- The Virus: The titular Entropy Plague. It rapidly ages everything it comes into contact with and, at the point of death, transforms them into the Sandmen, zombie-like monsters who can kill others with a touch.
- You Can't Go Home Again: As had been hinted at in "Prisoners of Fate", here Nyssa is prevented from ever getting back to her own universe or seeing her family again, remaining in E-Space for the rest of her life.
