Chekhovs Gun / Doctor Who

Due to Doctor Who spanning more than fifty years, there are a lot of Chekhovs Guns related to many episodes in the series.

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• In "Genesis of the Daleks", the Fourth Doctor is forced by the Time Lords to return to the moment of the Daleks' creation to destroy them before they are created — but when he actually gets the opportunity to do so, he decides not to on the grounds that 1) genocide is wrong, 2) the wars they eventually start will unite more races against them than otherwise, and 3) that without them some other race of space Nazis would rise up. All of these decisions come right back to bite him in the arse thirty (real life) years later, when the Daleks' retaliation against the Doctor's failed time erasure led to the Time War between the Time Lords and the Daleks, which created various obstructive alliances attempting to deal with the massive devastation this war caused (such as the Shadow Proclamation), and forced the Doctor to commit genocide against his own species as well as against the Daleks. It got Cosmic Retconned into him merely sealing Gallifrey away in its own dimension later on.
• The first victim of "The Robots of Death" was a meteorologist who was attacked when he was preparing to launch a meteorological helium balloon. It later turns out that the main villain had been reprogramming the robots to kill the humans on board the sandminer but was himself immune because the robots were programmed to recognise his voice and leave him alone. The solution, then, was to somehow change his voice so the robots would turn on him. Helium is good for that, and, as luck would have it, there was a ready supply on board.
• "The Five Doctors" introduces a gun that takes 30 years to go off. The High Council of Time Lords offers the Master a new regeneration cycle, showing they can do so. In "The Time of the Doctor", by which time the Doctor has run out of regenerations, he is given another cycle.
  • A minor gun introduced and used in the same episodes. The Third Doctor takes the Seal of the High Council from the Master. Later the Eleventh Doctor uses the seal to decode the signal on Trenzalore.
• "Warriors of the Deep" features intelligent reptiles as the Monster of the Week. Early in the story, a character identifies bottles of "hexachromite gas" as lethal to all reptile life, making the climax rather predictable.
• In "The Two Doctors", it's established early on that Oscar Botcheby collects moths, and to kill them he uses cyanide rather than ammonia. At the end of the story, the Doctor comes across the cyanide and butterfly net, and uses them to finish off the otherwise far stronger and deadly Shockeye.
• Bad Wolf, and the rest of the Arc Words.
• "Rose": The London Eye is seen near the beginning of the episode. At the climax, it turns out that it is the transmitter the Nestene Consciousness is using to activate the Autons, and its lair is underneath.
• "Aliens of London"/"World War Three": The flatulence produced by the Slitheen while they're in their human suits, technically called the "gas exchange". "Slitheen" is their surname, not their species, so the Doctor doesn't initially know where they're from. The farts, specifically their smell, provides the final clue for him to narrow their species down to one: Raxacoricofallapatorians. Knowing this allows the Doctor and company to use their Weaksauce Weakness against them.
• "The Empty Child"/"The Doctor Dances": When Rose meets Jack, she learns that his spaceship is full of nanogenes, microscopic medical robots. It later turns out that nanogenes from a related source are the cause of The Virus turning people into gas-mask zombies.
• "Boom Town" introduces a few:
  • The tribophysical waveform extrapolator, a forcefield-generating "pandimensional surfboard" that the villain was intending to use to escape Earth, is confiscated by the Doctor and comes in handy a few times.
  • Jack's handcuffs and Cardiff having a nuclear power plant were hugely important to spinoff Torchwood's series 2 finale, "Exit Wounds".
• "The Parting of the Ways" features two such instances. In the opening sequence, the TARDIS flies towards several missiles launched by the Dalek Emperor's ship, and it looks as though it's destroyed by the volley. However, the missile impact allowed the TARDIS to power the above-mentioned extrapolator and generate a forcefield that protects the TARDIS. When the TARDIS lands inside the Emperor's ship, the lone Dalek who transports inside is destroyed by Jack using the gun he improvised in the previous episode.
• "New Earth": The hospital's disinfectant lifts, akin to a human car wash, turn out at the climax to make a good dispersal system for the Doctor's cure cocktail.
• "Tooth and Claw":
  • The fact that Prince Albert had the Koh-i-Noor cut down, not to mention the diamond itself.
  • The telescope in Torchwood House's observatory, which as the Doctor notes isn't capable of actual stargazing.
• "The Girl in the Fireplace": The horse the Doctor finds wandering the spaceship, which he dubs "Arthur". At the climax, the Doctor rides the horse through a mirror/time portal to save the day.
• "The Satan Pit": A bolt gun with exactly one shot to use on one target. At the climax, Rose uses it to blow out a spaceship window so the villain is sucked out.
• "Army of Ghosts"/"Doomsday":
  • The 3-D glasses that the Doctor wears throughout the story, for no apparent reason until the climax. He hangs a lampshade on it, asking if anyone's going to ask why he's wearing them.
  • The magna-clamps that the Doctor and Jackie are shown in the first episode come back in the climax in an important way.
• "The Runaway Bride": Donna calling the Doctor a "Martian" leads to the Racnoss Empress underestimating him.
• "The Lazarus Experiment": During the conversation on the roof, Lazarus points out Southwark Cathedral and says that he and his family took refuge there during the Blitz. Guess where the episode's climax takes place.
• "42": It's mentioned early on that the spaceship runs off of illegal fusion scoop technology. At the climax, this tech turns out to be a major factor in why the ship is in trouble in the first place.
• "Blink": Larry gives Sally a list of every DVD with the Doctor's Easter Egg on it. This is a major clue as to the video's purpose and intended audience.
• The Chameleon Arch introduced in "Human Nature"/"The Family of Blood" shows up again in "Utopia", this time being used by the Doctor's old Arch-Enemy the Master, who used it to hide as a human after surviving the Time War.
• Subverted in "Last of the Time Lords". Early on, Martha explicitly introduces a gun that is believed to be the only thing that can kill a Time Lord. Later on the Master easily destroys the gun and it seems like all is lost — until Martha lampshades the ridiculousness of a plot hinging upon "a gun in four parts", then reveals her real plan.
• "Partners in Crime": The Adipose necklaces, and the fact that the Doctor and Donna both steal one. They're remote controls for inducing parthenogenesis.
• "Planet of the Ood": The villain, Mr. Halpen, is constantly drinking hair tonic given to him by an Ood slave. Later, we find out that the Ood have been feeding him a biological compound... which turns him into one of his own slaves.
• It's subverted in "The Sontaran Strategem"/"The Poison Sky". Part one goes to some trouble to point out Martha's engagement ring and her reluctance to use guns, leaving the audience to surmise that the absence of one or both of these will tip the Doctor off when she's replaced by an evil clone at the cliffhanger ending. Turns out it's actually neither; instead, the clone just smells wrong. Though, he mentions that this is one of MANY things...
• "The Unicorn and the Wasp": The Doctor gives Donna a large magnifying glass when asking her to search the bedrooms. About ten minutes later, she needs a way to defend herself against a giant wasp...
• "Silence in the Library"/"Forest of the Dead":
  • The Doctor notices that River's sonic screwdriver looks surprisingly like his own, and she later reveals that he gave it to her. It later occurs to him to wonder why he would do that, especially since his future self knew that she was going to die in the Library... turns out, he hid a telepathic recorder inside so he could save her life via Brain Uploading.
  • River tells the Doctor something: his own name.
• "Midnight": The Doctor's catchphrases "Allons-y" and "Molto bene" end up saving his life when the hostess hears the possessed Sky using them, giving her proof that the Eldritch Abomination has not jumped to the Doctor as most everyone else thinks, and leading her to sacrifice herself to throw Sky out.
• "The Stolen Earth"/"Journey's End":
  • The Shadow Proclamation had been mentioned several times^note "Rose", "The Christmas Invasion", "Partners in Crime" before it's revealed in this story to be a force of Space Police named after the document they enforce.
  • The Medusa Cascade and the spacetime Rift inside it were first mentioned in "Last of the Time Lords", where the Master says the Doctor closed the Medusa Rift single-handedly. The nebula was also mentioned in "The Sontaran Stratagem" and "Midnight" before finally appearing in this story. It turns out that it's where Davros and the Daleks have been hiding out, and the Rift there is a key part of their plans to destroy reality.
  • The first episode introduces the Osterhagen key, established as a rather obvious Chekhov's Gun; the finale also introduces two further devices with the potential to end Davros' plans, and characters threaten to use all three at the same time. The whole thing is subverted when the Daleks casually separate the characters from their respective doomsday devices. All seems lost until the real Chekhov's Gun goes off when Donna's Time Lord consciousness is awakened from the human-Time Lord metacrisis.
  • And let's not forget that the Doctor lost his hand in "The Christmas Invasion" which, after showing up a number of times in other episodes (including spinoff Torchwood) became the saving grace three seasons later in the climax.
• "Planet of the Dead" begins with Classy Cat-Burglar Lady Christina stealing a golden cup from a museum. Later on, it becomes essential to helping move the bus so it can escape back through the wormhole.
• "The Waters of Mars": Gadget the "funny robot" is turned into a high-speed Segway by the Doctor so he and Adelaide can escape some infected crew members. The Doctor later uses the souped-up robot to bring the TARDIS to him and the remaining crew.
• "The End of Time":
  • The Master's ring from "Last of the Time Lords", last seen being picked up by a mysterious woman. Russell T Davies originally planted this Gun with the intent of letting a future producer fire it, only to do so himself.
  • Quite literal: Wilf's revolver is essential to saving the day.
  • The Nuclear Bolt cabinet. Originally used by Joshua Naismith to power the Immortality Gate, it always requires one person to be inside it. Towards the end of the second episode, Wilf gets inside the cabinet to save one of Naismith's employees, but in the ensuing chaos the Nuclear Bolt overloads with radiation and the only way for the Doctor to save Wilf, and, presumably, everyone else is to take his place in the cabinet and absorb a massive amount of radiation, leading to his death and regeneration.
  • A three-year one — the Moment is first mentioned in this episode, but not until "The Day of the Doctor" do we learn what it is and why it's important.
• "The Eleventh Hour": Amelia gives the Doctor an apple with a smiley face carved into it. Later on, he uses it to convince 12-years-older Amy to help him.
• "Flesh and Stone":
  • The Doctor talks about the Byzantium's artificial gravity being the only reason everyone isn't falling to their deaths. At the end, the Angels become Hoist by Their Own Petard when they drain the ship of enough power that the gravity can't stay on, leading them to fall into the crack in time and be retgoned by it.
  • The Doctor walks away from Amy, earlier having lost his coat to a Weeping Angel. He then returns, warning Amy to keep her eyes closed, lest she allow the Angels access to the vision centres of her mind, and also telling her to remember what she told him when she was seven. The latter is shot entirely in close-up, and the Doctor's jacket-clad shoulder is only occasionally glimpsed (did Continuity miss something?). In fact, the Doctor who talks to Amy in the forest is actually the Doctor from the future, rewinding his timeline due to the events of "The Pandorica Opens"/"The Big Bang". The thing he told her when she was seven was a story about him and the TARDIS, meant to make her remember him at her wedding after he was erased, so he could be brought back into existence.
• "The Vampires of Venice": When it's revealed that Guido has barrels of gunpowder he nicked from work stockpiled in his apartment, you just know it's going to go boom later.
• "The Hungry Earth"/"Cold Blood":
  • Future Amy and Rory waving from across the way.
  • Ambrose's pile of weapons, one of which ends up spiking any chance at diplomacy.
  • The Meals on Wheels van is later used to capture Alaya.
• Amy's engagement ring. Rory leaves it in the TARDIS in "The Hungry Earth", then the Doctor finds it near the end of "Cold Blood" after Rory is erased from time and Amy has forgotten him. Amy then finds the ring while searching the Doctor's jacket for a pen in "The Lodger", and the ring is fired in "The Pandorica Opens" when Amy confronts the Doctor about why he's carrying the ring, and the Doctor gives the ring to a returned Rory so he can try and get Amy to remember him.
• River Song's lipstick offers a slight variation on this trope. Initially, her hallucinogenic lipstick is used by her to escape from jail in "The Pandorica Opens", it returns in "Let's Kill Hitler", this time as poison from the Judas Tree, which she has worn as part of her plan to kill the Doctor.
• The Teselecta from "Let's Kill Hitler" is revealed in "The Wedding of River Song" to have taken the place of the Doctor at Lake Silencio, allowing him to survive.
• "The Woman Who Fell to Earth":
  • While examining the alien pod, the Doctor identifies a recall circuit intended to send the occupant home. At the climax, she threatens to destroy it in a Hostage for MacGuffin situation.
  • During the climax, crane operator Karl is forced to crawl along the boom of his crane while fleeing from an alien headhunter. While he's doing so, he passes by a sign indicating a broken guardrail. He later kicks the alien through the chain serving as a replacement.
• "The Ghost Monument": Epzo has a rare, expensive cigar he intends to celebrate with if he wins the Galactic Relay, which lights with a snap of the fingers. It's eventually used to ignite a pocket of acetylene gas to destroy a number of Remnants (killer cloth, basically) who are attacking the group.
• "Rosa": Early in the episode, the Doctor mentions that she once left Elvis Presley a mobile phone. Later, she finds out where Rosa Parks gets off the bus by approaching her and claiming she's running a questionnaire for a raffle. Both the mobile and a fake raffle prove important when the Doctor and Yaz have to get a substitute bus driver out of the way so the right bus driver is on duty during Rosa's famous bus ride.
• "Arachnids in the UK":
  • The garbage Yaz's Conspiracy Theorist father collected as "evidence". Turns out it actually is, specifically of the illegal garbage dumping that created the Giant Spiders.
  • In a completely literal example, Robertson retrieves his bodyguard's pistol from his dead body, and later kills the mother spider with it.
• "Kerblam!": The bubble wrap inside every Kerblam! package is how the villain plots to kill thousands of innocent customers, via an explosive version.
• "The Witchfinders": Shortly after arriving in Bilehurst Cragg, the Doctor and company see a woman being tried for witchcraft in a ducking chair that the local landowner, Becka Savage, claims was created from the greatest tree on nearby Pendle Hill. It turns out that the tree, the fact that it was chopped down, and the chair made from its trunk are all crucial pieces to the puzzle of what's going on in the village.
• "Resolution": Aaron, Ryan's estranged dad, is trying to sell a fancy microwave that he's carrying around with him. At the climax, the microwave is stripped down for its components, which are used to destroy the Dalek's Improvised Armour.
• "Spyfall":
  • Among the spy gadgets C shows Graham and Ryan is a pair of shoes with built-in laser guns. They come in handy in part 2 when the companions are on the run and trying to defend themselves.
  • Visible in Barton's office is a wireframe humanoid statue in a glass case. When the Doctor encounters the "Silver Lady" again in 1834, she realizes its importance: it's a device built by the Master to make it easier for the Kasaavin to enter the main universe, and the Doctor takes the opportunity later to program it with some code that will sabotage the grand plan.
  • At the Alexandria Gallery, among the inventions being shown off are a steam-powered ball-bearing machine gun and grenades for home defence. When the Master is distracted lording it over the Doctor, Ada goes for the weapons and attacks him with them, injuring him and forcing him to retreat.
• "Orphan 55": Early on, the Doctor saves Ryan from a hopper virus that he caught from a vending machine, catching it in a bag. Hopper viruses also have the ability to upgrade syrillium 3 fuel into syrillium 4, which is needed to power the off-planet teleport.
• "Nikola Tesla's Night of Terror": When Tesla and Yaz get kidnapped by teleport, the Doctor uses her own teleport bracelet and power source to go rescue them. At the climax, the Doctor tricks the Skithra Queen into seizing the teleport bracelet in order to forcibly return her to her ship.
• "Fugitive of the Judoon": Conspiracy Theorist Allan attempts to give Ruth a dossier he's compiled on her husband Lee as he believes him to be up to something. He later gives the folder to the Judoon when he believes Lee to be the fugitive they're hunting.

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