The one where the Doctor takes a shower.
"Spearhead from Space" was the first Doctor Who story to be broadcast in colour, as well as the beginning of a long story arc (ending with "The Three Doctors") that takes place exclusively on Earth. Budgetary restraints now meant that the setting would be "contemporary Earth" for the forseeable future, with the Doctor's "exile" being a case of Real Life Writes the Plot. With the Third Doctor being confined to life with UNIT, the show's atmosphere changed completely and started drawing much inspiration from that other famous Sydney Newman show: The Avengers.
The TARDIS materialises (in colour, no less!) and a tall, white-haired man steps out and collapses.
UNIT troops arrive and find the man, taking him off to hospital, where he spends some time in a coma. Brigadier Lethbridge-Stewart arrives and, after hearing that the man has two hearts and was found near a police box, concludes that it's probably the Doctor. When the Doctor wakes up — still with his previous personality — he's not at all happy at having been forcibly regenerated, but has to admit that his new face is rather dignified. One gratuitous shower scene later, he's stolen some clothes and a car (from the "Doctors only" car park) and is on his way to UNIT, where scientist Liz Shaw is trying to make sense of the mystery of the day. An employee of the local plastic factory is claiming that he's seen a walking mannequin. And there's a meteorite shower on.
The new Doctor is still suffering from amnesia (caused by both his execution and the Time Lords having removed large chunks of his memory pertaining to TARDIS operation) and tries to get used to his new personality while he investigates the strange meteorite shower. He discovers that the meteorites are in fact hollow globes containing the Nestene Consciousness, another disembodied alien intelligence (see "The Abominable Snowmen"), which uses plastics to achieve its ends. In one classic scene, shop dummies come to life, smashing through the shop windows and firing guns concealed in their hands, spreading panic — but the Nestene Consciousness's real plan is to create plastic replicas of world leaders and take over the Earth.
The Doctor and Liz manage to repel the Consciousness back into space and the Doctor agrees to sign on as UNIT's "Scientific Advisor" for the duration of his exile, signing the paperwork as Dr. John Smith. In exchange, UNIT will give him technical facilities to try and repair the TARDIS (which has been deactivated by the Time Lords), and the use of a vintage car just like the one he had earlier appropriated from the hospital.
- Answer Cut:Corporal: Is he dead, sir?
[cut to the hospital, some time later]
Dr. Henderson: No.
- Arm Cannon: The Autons love their folding/unfolding wrist guns.
- Artistic License Chemistry: In the scene where Liz describes how the alien plastic differs from anything known on Earth, everything she mentions is actually a real characteristic of Earthly plastics — but by the time she's finished, she's effectively said that it lacks every characteristic necessary to defining a substance as being a plastic.
- Art Shift: As mentioned, the only episode of Doctor Who shot entirely on film.
- Aside Glance: When the Brigadier and the other members of UNIT leave the Doctor alone and unguarded in his hospital bed, he gives the camera a short, conspiratorial look.
- Attack of the Killer Whatever: Attack of the killer 1970s shop window mannequins!
- Attention Deficit... Ooh, Shiny!: The Doctor finds the shell of a Nestene Consciousness much more interesting than his conversation with the Brig, and breaks into talking about it mid-sentence.
- Bizarre Alien Biology: Dr. Henderson spends most of the first episode trying to comprehend a creature that has two hearts, unidentifiable blood, a pulse rate of 10 per minute and a flatline EEG.
- Bound and Gagged: The Doctor is kidnapped by a raiding party of Autons. He escapes. In a wheelchair.
- Brief Accent Imitation / Continuity Nod: With one of his first lines, Pertwee does his best Patrick Troughton imitation: "That's not me 'tall!"
- Can't You Read the Sign?: Played with — in the hospital, the Doctor walks right past a "Doctors Only" sign.
- The Charmer: The Doctor quickly seduces Liz and flirting her into getting his TARDIS key for him (though she loses interest when she realises he's tricked her).
- Combat Tentacles: The Nestene attacks the Doctor with these.
- Corpsing: When the Doctor turns up the charm and swaggers across the room to Liz, Caroline John gives a rather helpless little giggle.
- Could Have Been Messy: The Doctor gets shot in the head, and VERY narrowly dodges a bullet to the brain in Episode 1.
- Creator Cameo: Producer Derrick Sherwin plays the UNIT guard the Doctor gets past.
- Creepy Doll: Part 2 has a scene in a creepy doll factory.
- Darker and Edgier: Part 2 has a guy whose face gets smashed into a windshield. This trope would become a staple of Robert Holmes episodes.
- Distressed Dude: In the very first episode of his very first story, the Third Doctor gets gagged and kidnapped by the Autons, who mistakenly think the reason UNIT is interested in him is because he has information on where the Auton Spheres landed.
- Doppelgänger: The Auton facsimiles of key authority figures.
- Everything's Squishier with Cephalopods: The Nestenes create "a life-form perfectly adapted for survival and conquest on this planet"; it's basically an enormous squid.
- Family-Unfriendly Violence: There's a car accident when a man avoids running down an Auton and instead goes into a tree. We see a shattered windscreen covered in blood.
- Fanservice: The Doctor's Shower Scene. Tempered somewhat by the tattoo on Jon's arm and fandom's attempts to explain it away. (It's generally accepted that the tattoo is a Time Lord-administered "criminal mark" that remains in place until the Doctor's exile is lifted.)
- Fingore: A meta-example: Derek Smee, who played Ransome, gashed his left index finger pretty badly while climbing over the barbed wire while on location shooting the "infiltration into the plastics factory" scene. He insisted on the doctors administering first aid and placing a flesh-coloured bandage over the finger so he could return to finish the day's shoot. Afterwards, he was immediately rushed to a hospital for surgery. Scenes shot subsequently show him with a bandage on the finger and favoring it while climbing stairs.
- Had the Silly Thing in Reverse: Inverted when the Doctor makes off with the surgeon's car — he needs to reverse to get out of the parking space, but starts going forward first.
- Jon Pertwee claimed that he damaged the car slightly during this sequence with that gag — he had accidentally thrown the car into the wrong gear and wrenched the transmission.
- He Knows Too Much: Ransome.
- Hero Stole My Bike: The Doctor "borrows" a vintage car and takes a liking to it. UNIT does return the car to its rightful owner, but they get the Doctor another one a lot like it.
- Hidden in Plain Sight: The waxworks museum where evil plastic Doppelgangers of government officials are held. And where their victims are frozen in place.
- Immune to Bullets: The Autons.
- Innocent Innuendo: It's hard to tell whether it was this or an Accidental Innuendo, but when the actual doctor meets with his superior in the restroom and they discuss the Doctor, he asks if his superior wants to review the Doctor's charts "before you examine him." Little do they know, the Doctor is showering with his back to them, in plain view. "Examine" indeed.
- Laser-Guided Amnesia: The Doctor is not happy when he realises that his amnesia isn't just regeneration sickness, and that the Time Lords have messed with his brain.
- Murderous Mannequin: The Autons. Shop window dummies coming alive to stalk the streets is one of the most iconic moments of classic Who.
- Mr. Smith: The Doctor's chosen "alias". The Brigadier is not impressed.
- The Nth Doctor: Jon Pertwee makes his debut as the Doctor.
- Nobody Here but Us Statues: The Doctor and Liz hide in the waxworks museum in this way.
- Non-Indicative Name/Line-of-Sight Name: The name "Auton" comes from Auto Plastics, the factory where they were first made.
- To disguise non-union actors being used as Autons, internal BBC documents credited the extras as "Ivan Orton." Say it fast.
- Obscured Special Effects: The BBC didn't have the budget at the time to show the dummies breaking the glass, so instead we just cut to the faces of shocked onlookers while the sound effect plays. This was gleefully played with in the New series Auton episode ("Rose"), in which they did have the budget to break the glass, and so we got several pornographic, slow-motion shots showing the Autons doing it again and again...
- Reasonable Authority Figure: General Scobie is rather dubious of his claims when he talks to him on the phone, but agrees that it sounds serious and will give the Brigadier his whole support. Then he puts down the phone and answers the door...
- Retool: A black-and-white series based on travelling throughout time and space becomes a colour series set entirely on 1970s (or was it the eighties...) Earth with a largely new regular cast.
- Resurrection Sickness: The newly-regenerated Doctor spends the first episode in hospital in a somewhat drowsy condition.
- ShoutOut: "Smith. Doctor John Smith.". Jon Pertwee was after all a great 007 fan, and this reference would only be the first of many during his tenure. Made awesome in hindsight by the fact that Pertwee was friends with 007 creator Ian Fleming, and once worked in the British Secret Service with him during WWII.
- Shower Scene: The Doctor takes a nice long shower, which to this day is still the largest amount of Doctor nudity the show has seen.
- Singing in the Shower: The Doctor warbles out a tune while taking a shower.
- Smash Cut: The Auton rampage in Part 4 uses a literal "smash" cut to suggest that the mannequins have broken through the shop-window.
- Sound-Only Death: The Seeley's dog is apparently killed by an Auton, or at least spooked off. The animal is heard barking offscreen, as its mistress berates it to be quiet; this is followed by a yelp, then silence.
- This Was His True Form: The Auton facsimiles revert to the default blank-faced Auton form when they're deactivated.
- Took a Level in Dumbass: While UNIT used sensible tactics, like grenades and rockets, on the Immune to Bullets Cybermen when we last saw them, they now keep shooting the equally bullet-immune Autons without result.
- Tracking Device: After UNIT takes the TARDIS into protective custody while the Doctor's in hospital, the Doctor uses a homing device to find the TARDIS and therefore UNIT HQ.
- The Autons can also track the orbs containing the Nestene consciousness, as long as they aren't shielded too much.
- Wax Museum Morgue: Likely a homage to this trope when government officials who've been replaced by Auton replicants are 'stored' under hypnosis at Madame Tussands.
- Wham Episode: The series is now in colour, the entire starring cast is reset, the Doctor is suddenly on a much more violent adventure, it's revealed that the Doctor has two hearts, and the space race suddenly has a much more chilling consequence. It also satisfies the "game changing" aspect of the trope by changing the format of the series, with the Doctor now exclusively Earth-based and working for UNIT, rather than traveling on his own.
- Wiper Start: When the Doctor tries to drive a car, he finds the horn before the starter, and starts in forward gear when he needs to reverse. (But no funny business with the wipers, because it's a veteran model that doesn't have any.)