History Recap / DoctorWhoS18E7Logopolis

* EverybodyDiesEnding: Every single member of the eponymous planet and the ''Doctor'' die (he gets better, of course). Plus, approximately a quarter of '''the universe''' gets destroyed. ''By accident.'' Special points for this explicitly including all of the surviving characters from the previous story, except Nyssa. The only survivors are the companions (Nyssa's entire family dies in this story, and Tegan's only on-screen family member gets murdered) and the Master. And, well, the Fifth Doctor is created at the end of this story.

* GoodWithNumbers: Logopolis is this trope in ''city form''.

* ShownTheirWork: Writer Creator/ChristopherHBidmead seems to have done more reading up on computers than on entropy. The monks of Logopolis work and communicate in hexadecimal (base 16), just like real computers. At one point, the Doctor mentions "bubble memory"--which is real, and was the subject of a lot of research in 1981.
* SpannerInTheWorks: The Master doesn't actually ''plan'' on wiping out part of the universe or destroying Logopolis. See also GoneHorriblyRight.

* TheVoiceless: Other characters refer to the Watcher speaking but even when he's having a dialogue scene with someone it's always shot from a distance and inaudible.

* EverybodyDiesEnding: Every single member of the eponymous planet and the ''Doctor'' die (he gets better, of course). Plus, approximately a quarter of '''the universe''' gets destroyed. ''By accident.'' Special points for this explicitly including all of the surviving characters from the previous story, except Nyssa. The only survivors are the companions (Nyssa's entire family dies in this story and Tegan's only on-screen family member gets murdered) and the Master. And, well, the Fifth Doctor is created at the end of this story.

* FinaleCredits: Since this is Creator/TomBaker's swan song, his face is electronically obscured during Episode 4's ClosingCredits. (Peter Davison is also credited, marking the first time an incoming Doctor received a credit for a regeneration sequence.)

* MyBrainIsBig: The inhabitants of Logopolis.

* RealityWarping: The Logopolitans are seemingly capable of arbitrary reality warping by meditating on the equations that physically describe the universe, or objects within it.

* EvilHand: Nyssa's bracelet.

* KillEmAll: Every single member of the eponymous planet and the ''Doctor'' die (he gets better, of course). Plus, approximately a quarter of '''the universe''' gets destroyed. ''By accident.'' Special points for this explicitly including all of the surviving characters from the previous story, except Nyssa. The only survivors are the companions (Nyssa's entire family dies in this story and Tegan's only on-screen family member gets murdered) and the Master. And, well, the Fifth Doctor is created at the end of this story.

* TheWatcher: Played straight with The Watcher, who watched the Doctor and his companions throughout the serial, and was revealed to be a manifestation of the Fourth Doctor, which Foreshadows his regeneration into the Fifth Doctor.

!!Tropes
* AMillionIsAStatistic: The story involves the destruction of one quarter of the entire universe; by even the most conservative estimates that's a single-episode death toll expressible only in standard notation. And not only does nobody seem to care (including one character whose home ''intergalactic supercluster'' was destroyed [[note]] He also killed her Dad [[/note]]), the villain responsible gets the full DracoInLeatherPants[[invoked]] treatment despite being responsible for one of the most horrifying massacres in the series. (Though in all fairness, [[MikeNelsonDestroyerOfWorlds it was an accident this time]].)

The TARDIS soon materializes on the dusty, cave-pocked planet of Logopolis, home of the maths monks. It is, we learn, an enormous analogue of a computer. A task is broken down into a series of calculations; each monk performs a single calculation and passes the result on to the next monk over. Operations are overseen by the Monitor (get it?) in a sort of large workroom. After reuniting with Nyssa, who is deposited on the planet by the mysterious white Watcher, the TARDIS crew finds the Monitor. He explains that the universe is actually well past its sell-by date, and is kept from flying apart into chaos only by the power of their mighty minds, as they use their "block transfer computations" to keep the universe together.

* {{Tulpa}}: The Watcher turns out to be an "intermediate stage" between the Doctor's Fourth and Fifth selves, with the actual regeneration depicting Four turning into the Watcher, then into Five. The Watcher apparently had some level of independent consciousness and actively contributed to the events that led to the Fourth Doctor's demise. Exactly how the Watcher came into being is left unexplained, though a brief cameo in the Tenth Doctor short story ''The Advent of Fear'' implies that any incarnation of the Doctor can encounter a Watcher in the twilight of their life.

* EnemyMine: The Doctor and the Master must work together to save all of existence.
* EstablishingCharacterMoment: When Tegan gets lost inside the TARDIS, instead of being astonished by it, she goes and looks for someone to yell at.

The one with the cardboard cutout Master. And Four [[DeathByFallingOver had a big fall.]]

The sonorous ''bong'' of the TARDIS's cloister bell (its OhCrap signal) makes itself heard, so the Doctor and Adric eventually find their way through the nesting [=TARDISes=] back to the outside world. The Doctor suspects that the multiple [=TARDISes=] thing is yet another devious trick of the Master, and dematerializes at once, bound for Logopolis. Tegan, by now extremely confused, finds her way back to the control room and demands to know what the hell is going on and to be put back in 1981 ''right now'', please -- rather angrily, true, but then again "Sorry I'm late, I accidentally stepped into a spacetimeship that looked exactly like a police box and ended up travelling the universe in the company of an alien with an enormous scarf and a swotty maths geek from another dimension" is, as excuses go, pretty lousy. Especially for one's first day on the job.

After a chat with the Watcher inside the deepest parts of the TARDIS, Adric is able to follow the Doctor, and the ducklings cause enough of a distraction to allow the Doctor to muck up the Master's plans. Fortunately, this leaves the larger part of the universe intact. Unfortunately, the Master and the Doctor break out into a fight on top of the Pharos Tower. The Doctor sees visions of his worst enemies attempting to kill him. Clashing on the gantry, the two Time Lords wrestle to the floor. The Master gets up first and bolts to the controls of the telescope. He tilts the radar dish until the gantry completely flips over. Grabbing a cable and desperately trying to hang on to it, the Doctor sees several visions of his various enemies taunting him. Gradually, his strength begins to fail him, and finally his grasp slips. The cowardly Master runs away sniggering in his TARDIS after this cheap act of murder.

* ContinuityCavalcade: Before the Fourth Doctor regenerates, he sees images of his enemies and former companions, all shown by clips from previous stories - [[Recap/DoctorWhoS14E3TheDeadlyAssassin the Master]], a [[Characters/DoctorWhoDaleks Dalek]], [[Recap/DoctorWhoS12E5RevengeOfTheCybermen the Cyber-Leader]], [[Recap/DoctorWhoS16E2ThePiratePlanet the Captain]], [[Characters/DoctorWhoDavros Davros]], a [[Characters/DoctorWhoSontarans Sontaran]], [[Recap/DoctorWhoS13E1TerrorOfTheZygons a Zygon]], [[Recap/DoctorWhoS16E6TheArmageddonFactor the Black Guardian]], the Sarah-Jane Smith, Harry Sullivan, the Brigadier, Leela, [[Characters/DoctorWhoK9 K-9]] and [[Characters/DoctorWhoRomana Romana]].

* [[DroppedABridgeOnHim Dropped Him Off A Radio Tower]]: What causes the Doctor's regeneration.

* {{Tulpa}}: The Watcher turns out to be an "intermediate stage" between the Doctor's Fourth and Fifth selves. The Watcher apparently had some level of independent consciousness and actively contributed to the events that led to the Fourth Doctor's demise.

* WetwareCPU

Written by Creator/ChristopherHBidmead. This serial first aired February 28-March 21, 1981.

* AMillionIsAStatistic: The story involves the destruction of one quarter of the entire universe; by even the most conservative estimates that's a single-episode death toll expressible only in standard notation. And not only does nobody seem to care (including one character whose home ''intergalactic supercluster'' was destroyed [[note]] He also killed her Dad [[/note]]), the villain responsible gets the full DracoInLeatherPants treatment despite being responsible for one of the most horrifying massacres in the series. (Though in all fairness, [[MikeNelsonDestroyerOfWorlds it was an accident this time]].)

* ShootTheShaggyDog: A significant proportion of the ''entire universe'' is destroyed by entropy. Although insignificant on the scale of such an unbelievably cataclysmic event, the region destroyed includes the Traken Union, thereby almost immediately rendering all the events, people and struggles in ''[[Recap/DoctorWhoS18E6TheKeeperOfTraken The Keeper of Traken]]'' (the previous connected story) dramatically null, void and pointless. (The fact that this is only obvious when you [[FridgeLogic stop and consider it]] suggests that this was a side-effect rather than dramatic intent on the part of the writers).

* FemmeFatalons: The Watcher sports pointy claws for some reason.

* GeorgeLucasAlteredVersion: In the 2019 Blu-ray release of Season 18 viewers are given the option to watch an updated version of this episode, which features new special effects, replacement of the model footage of the radio telescope in Episode 4 with live-action footage of the telescope at Jodrell Bank, and an additional scene where, after watching his hand slip away from the strut as we do in the original, we see the Fourth Doctor plunging to his death.

** Likewise, the episode (beyond its central regeneration) arguably serves as a backdoor pilot for Creator/PeterDavison's run through applying numerous further changes to the show's cast across its four episodes, most notably through acting as both the introduction of Tegan Jovanka (the primary companion for the vast majority of the Davison era) and the full debut of Creator/AnthonyAinley's rendition of the Master, who would secure a recurring antagonistic role throughout the remainder of '80s Who.

* WhamEpisode: Creator/TomBaker was the most popular Doctor the classic series ever had, so his departure after seven seasons into the (then) youngest Doctor actor ever was a Wham; Baker had been associated with the role for so long that there were fears about whether an entire generation of children who had never known another Doctor would be able to accept another one. Likewise, the episode (beyond its central regeneration) arguably serves as a backdoor pilot for Peter Davison's run through applying numerous further changes to the show's cast across its four episodes, most notably through acting as both the introduction of Tegan Jovanka (the primary companion for the vast majority of the Davison era) and the full debut of Anthony Ainley's rendition of the Master, who would secure a recurring antagonistic role throughout the remainder of '80s Who.

* WhamEpisode: Creator/TomBaker was the most popular Doctor the classic series ever had, so his departure after seven seasons into the (then) youngest Doctor actor ever was a Wham; Baker had been associated with the role for so long that there were fears about whether an entire generation of children who had never known another Doctor would be able to accept another one. Likewise, the episode (beyond its central regeneration) arguably serves as a backdoor pilot for Peter Davison's run in numerous other facets, most notably through acting as both the introduction of Tegan Jovanka (the primary companion for the vast majority of the Davison era) and the full debut of Anthony Ainley's rendition of the Master, who would secure a recurring antagonistic role throughout the remainder of '80s Who.

The universe nearly ends and Creator/TomBaker leaves ''Doctor Who'' after an amazing-- and still unmatched-- seven years and forty-one ([[Recap/DoctorWhoS17E6Shada or forty-two]]) serials. We're not sure which is more important at this stage.

* WhamEpisode: Creator/TomBaker was the most popular Doctor the classic series ever had, so his departure after seven seasons into the (then) youngest Doctor actor ever was a Wham; Baker had been associated with the role for so long that there were fears about whether an entire generation of children who had never known another Doctor would be able to accept another one. Likewise, the episode (beyond its central regeneration) arguably serves as a backdoor pilot for Peter Davison's run in numerous other facets, most notably through acting as both the introduction of Tegan Jovanka (the primary companion for the vast majority of the Davison seasons) and the full debut of Anthony Ainley's rendition of the Master, who would secure a recurring antagonistic role throughout the subsequent seasons (and the remainder of '80s Who, save for most of the McCoy era, at large).

* WhamEpisode: Creator/TomBaker was the most popular Doctor the classic series ever had, so his departure after seven seasons into the (then) youngest Doctor actor ever was a Wham; Baker had been associated with the role for so long that there were fears about whether an entire generation of children who had never known another Doctor would be able to accept another one. Likewise, the episode (beyond its central regeneration) arguably serves as a backdoor pilot for Peter Davison's run in numerous other facets, most notably through acting as both the introduction of Tegan Jovanka (the primary companion for the vast majority of the Davison seasons) and the full debut of Anthony Ainley's rendition of the Master, who would secure a recurring antagonistic role throughout the subsequent seasons.

* WhamEpisode: Creator/TomBaker was the most popular Doctor the classic series ever had, so his departure after seven seasons into the (then) youngest Doctor actor ever was a Wham; Baker had been associated with the role for so long that there were fears about whether an entire generation of children who had never known another Doctor would be able to accept another one.

* ShownTheirWork: Writer Christopher H. Bidmead seems to have done more reading up on computers than on entropy. The monks of Logopolis work and communicate in hexadecimal (base 16), just like real computers. At one point, the Doctor mentions "bubble memory"--which is real, and was the subject of a lot of research in 1981.

Adric and Nyssa, meanwhile, can only watch helplessly, as the encroaching entropy wipes out a potion of the universe and of couse, because Nyssa's day can [[SarcasmMode only gets better]], it includes Traken.

* ContinuityCavalcade: Before the Fourth Doctor regenerates, he sees images of his enemies and former companions, all shown by clips from previous stories - [[Recap/DoctorWhoS14E3TheDeadlyAssassin the Master]], a [[Characters/DoctorWhoDaleks Dalek]], [[Recap/DoctorWhoS12E5RevengeOfTheCybermen the Cyber-Leader]], [[Recap/DoctorWhoS16E2ThePiratePlanet the Captain]], [[Characters/DoctorWhoDavros Davros]], a [[Characters/DoctorWhoSontarans Sontaran]], [[Recap/DoctorWhoS13E1TerrorOfTheZygons a Zygon]], [[Recap/DoctorWhoS16E6TheArmageddonFactor the Black Guardian]], the Sarah-Jane Smith, Harry Sullivan, the [[Characters/DoctorWhoBrigadierLethbridgeStewart Brigadier]], Leela, [[Characters/DoctorWhoK9 K-9]] and [[Characters/DoctorWhoRomana Romana]].