The one where Harry spends a scene coming close to lecturing the viewers about those rocks.
The Doctor decides to do a favour for the people of the Nerva Space Station, and beams down to Earth to check out if it's safe to re-colonize. Landing on an old teleport pad, the Doctor, Sarah and Harry find a rather... smooth and barren Earth. Nevertheless, the Doctor claims they're actually in the centre of what used to be London, and encourages Harry and Sarah to go sightseeing while he does some work on bringing the teleporter back up to spec.
Harry promptly falls down a crevasse— which, in his defense, had been deliberately concealed with branches. Faced with this worrying evidence that the planet isn't as deserted as it's supposed to be, Sarah hurries back to the Doctor, only to find that he's disappeared. While she's away, Harry finds and begins following a passage that seems to lead out of the crevasse, so when Sarah returns, he's disappeared too.
The Doctor has been captured by a group of astronauts from one of Earth's interstellar colonies, which are apparently still going strong. They're in an unfriendly frame of mind: they landed in response to a distress call, only for their ship to be destroyed and several of their crewmates to mysteriously disappear. As a stranger wandering about without a plausible explanation for his presence, the Doctor immediately becomes the focus of their suspicions.
Meanwhile, Sarah meets the setter of the pit-trap, who saves her from being captured by a robot that's roaming the area (which is what he set the trap for). His name is Roth, and he's one of the missing astronauts. He's also clearly deranged, and Sarah is not sure how much to believe of his story, which involves being captured by the robot, tortured by its alien master, escaping, and being afraid to return to his crewmates because one of them, Vural, was also captured and is now serving the alien.
Though he won't go near his crewmates, he shows Sarah where to find them, and creates a distraction while she rescues the Doctor. The Doctor, Sarah, and Roth go back to the crevasse to look for Harry, where the Doctor goes down into the crevasse. While he's down there, Sarah and Roth are captured by the robot and taken to the alien.
The alien is, in fact, Field Major Styre of the Sontaran Military. Sarah is terrified, recognizing him as a Sontaran like Linx from her first story (and briefly thinks that he is Linx, because the Sontarans are a clone species who all look alike, not that the radically different mask Sontaran actor Kevin Lindsey wears in this story indicates that). Styre shoots Roth, and takes Sarah off to be tortured.
Harry has been watching this, and tries to rescue Sarah, only to find she's imprisoned behind some kind of forcefield. He heads off to find the Doctor, and along the way finds another of the missing astronauts, who has been chained up and deprived of water for days. Harry tries to help him, but he dies rather gruesomely of thirst shortly afterwards.
The Doctor finds the same passage out of the crevasse Harry used and, retracing Harry's path, finds Sarah being exposed to a device that forces her to hallucinate her fears. He's able to disable the force field and free Sarah from the device, but is shot and left for dead by Styre.
When the Doctor recovers, he collects Harry and Sarah and they head off to see what Styre is up to now.
The remaining three astronauts are captured by Styre's robot, and it's revealed that astronaut Vural really is working with Styre in hopes of sparing his own life. Which doesn't work, as Styre now tortures all three astronauts. Because he needs to figure out the various resistances of humankind in order to report back to Sontaran Command on if Earth can be taken over easily. You know, despite it being abandoned. The Doctor shows up and challenges Styre to mortal combat, and the pair begin to fight. Vural dies trying to help the Doctor.
Sadly, the Doctor doesn't beat Styre in combat, but he buys enough time for Harry to mess around with Styre's space ship. Battered but not defeated, Styre flees back to his ship to feed on energy... only to melt and explode from whatever Harry messed with. The Doctor then speaks directly to Sontaran Command and plays a few mind games with them — convincing the Sontarans that the Earth isn't something to mess with right now. The day saved, despite only two of the original nine astronauts remaining alive, the Doctor and companions hop back in the teleportation pad to go back to Nerva Station and let them all know that the Earth is just fine and dandy now.
This is the only two-part adventure in The '70s, which wouldn't happen again until "Black Orchid" in 1982. It's also one of very few to not involve the TARDIS even by implication: the Doctor and companions arrive and leave by transmat beam.
- All There in the Script: According to the script, Styre's first name is Weam.
- Appeal to Audacity: The first astronaut to start considering that the Doctor might be telling the truth does so on the basis that his story is too ridiculous to be a plausible lie.
- Are These Wires Important?: Harry defeats Styre by yanking some parts out of his ship just before Styre needs to use it, causing a malfunction.
- Art Shift: The second Doctor Who story shot entirely on location and the first of such to be shot entirely on video. As a result of this then-unusual combination, many scenes look like they were shot more for a PBS documentary than for a science fiction story. At one point, a shot features Harry precariously climbing a mound of rocks in the camera's direction, as if he's preparing to introduce a historical lecture.
- Berserk Button: The first time in the Fourth Doctor's tenure that he seems genuinely furious. The Doctor has been stable with everything else that had happened in the story so far. But what sets him off? Discovering Styre has been mentally torturing Sarah Jane.Styre: Very touching...Doctor: (bares his teeth and rounds on Styre) You unspeakable abomination!!
- Bluffing the Advance Scout: The basis of the plot. The Doctor and his friends have to defeat advance scout Styre and persuade the Sontaran invasion fleet that the humans are too much trouble to bother with.
- Bottle Episode: The whole story, in addition to being two episodes long and shot on videotape to save money, takes place entirely on a hillside. There are four guest stars in recycled space suits, one of whom is even played by Tom Baker's Stunt Double Acting for Two, and two Sontarans (recycled costumes). There's a Tin-Can Robot and some props recycled from the Sontaran spaceship sets of previous Sontaran stories. It also has no scenes in the TARDIS. This was also Tom Baker's first story in which he could adjust to the role. The cost-saving genius was that is was made using the location filming allocated to the four-parter it was produced alongside, resulting in two stories shot for the price of one. This practice was later fine-tuned under the last producer of the classic series, John Nathan-Turner, and was the only method of production for the shortened, No Budget last three seasons under the Seventh Doctor.
- But What About the Astronauts?: They're visiting the now (mostly?) empty Earth.
- Call-Back: Sarah assumes that Styre is Linx.
- Characterization Marches On: Sarah Jane is rescued from the fear experiment before hallucinating the clown.
- Chromosome Casting: Sarah is the only female character in the story.
- The Corpse Stops Here: The Doctor investigates a noise and finds the body of one of the astronauts who has been killed by Styre's robot. Being found by the still-warm body of their colleague makes a bad beginning to his attempt to persuade the surviving astronauts he's a friend.
- Coldblooded Torture For Science!: Styre's experiments.
- Denied Food as Punishment: The victim that Harry finds tied in shackles onto a rock. Being denied water, he lasts nine days and seven hours before dying of thirst.
- Drowning Pit: One unseen victim lasts less than 3 minutes in submersion before asphyxiation.
- Mind Probe: A device attached to Sarah's head makes tests her resistance to fear. She sees three hallucinations: a snake, a falling rock, and porridge devouring her leg.
- Squashed Flat: The gravity bar Erak and Krans hold above Vural is used to test the human body's resistance to compressibility. It also doubles as a test of human determination and willpower, as he observes how long Erak and Krans can keep the ever-increasing weight from killing Vural even after he betrayed them and even as the bar exceeds the limits of what their bodies can support.
- Creator Provincialism: Despite being so far in the future that they could have claimed to be anywhere and got away with it, the story is set in London yet again.
- Deadpan Snarker: After Styre casually kills Roth."Why did you make that disagreeable noise?"
- Deflector Shields: Styre sets one up to prevent anyone from releasing Sarah Jane.
- Evil Cannot Comprehend Good: Styre is baffled as to why Erak and Krans struggle to save Vural from being crushed by the gravity bar, repeatedly stressing that he's a traitor and doesn't deserve their mercy. Similarly, he's confused as to why Sarah makes a "disagreeable noise" (i.e. a bloodcurdling scream) upon seeing a man shot dead.
- Fake Shemp:
- Tom Baker shattered his collarbone during filming, requiring all the action stuff afterwards to be filled in by a double in a wig.
- The Doctor's fight with Styre had neither character played by his main actor (Kevin Lindsay was unable to perform action scenes because of a heart condition).
- Forever War: The Sontarans want to take over the Earth to gain a strategic foothold in the interminable war against the Rutan Host as previously mentioned in "The Time Warrior". Bear in mind that story is set in The Middle Ages and this one in the very distant future.
- Gravity Is a Harsh Mistress: The gravity bar runs either on variable gravity, or complete disregard for the conservation of mass.
- Given the widespread use of Artificial Gravity on the series, it's likely the same technology was involved.
- Honor Before Reason: Styre can't refuse a duel with the Doctor, even though it allows Harry and Sarah Jane to release his prisoners.
- Hypocritical Humor: The Doctor is showing Harry the piece of metal that saved him when he was shot by Styre:Doctor: Piece of the locking system on Nerva. Popped it into my pocket.Harry: Fortuitous.Doctor: Foresight. You never know when these bit and pieces will come in handy. Never throw anything away, Harry. (Throws it away) Now, I remember jotting some notes about Sontarans in my diary... (Searches himself)... It's a mistake to clutter your pockets, Harry.
- I'm Melting!: Styre's head deflates like a kickball when he dies.
- Irony: The people in suspended animation on the Nerva assumed they were the last hope for humanity. Turns out humanity survived the solar flare disaster and went on to forge a galactic empire.
- Let's Split Up, Gang!: The Doctor encourages Harry and Sarah to wander around and look at future Earth while he's repairing the transmat station. Then Harry falls down a large hole, and when Sarah goes back to get the Doctor he's disappeared...
- Look Behind You:
- Some of the astronauts get caught by the robot because when the Doctor tries to warn them it's approaching they think it's a trick to let him get away.
- Later, the Doctor does get away from Styre after tricking him into turning around by acting as if Harry is sneaking up behind him.
- Lost Colony: What the astronauts think Nerva is.
- Mind Rape: Styre captures Sarah and tortures her by making her hallucinate a snake crawling up her leg. (The subtext to that is fairly obvious, right?) The Doctor absolutely loses it when he finds out what Styre did to her, diving at him with every intention of murdering him on the spot even though the Doctor is unarmed and Styre is a solidly-built warrior-race alien in heavy armour. He even actually growls at Styre.
- Nobody Calls Me "Chicken"!: How the Doctor tricks Styre into fighting him while Harry screws around with his golf ball spaceship. Being a Sontaran, Styre accepts without thinking of the possibility of him being distracted.
- People in Rubber Suits: The Sontarans, redesigned from their first appearance now mostly look like... happy smiling teddy bears? It is a bit weird when Sarah claims Styre and Linx are identical when the two costumes used in the different stories look so very different.
- Plot-Powered Stamina: For all of Styre's mocking of how fragile humans are (including dying from starvation/dehydration after only a few days), he doesn't have the endurance to handle a melee fight with the Doctor lasting a few minutes.
- Pocket Protector: When Styre shoots the Doctor, he is saved by a lump of metal in his pocket that he put there in the previous adventure.
- Ragnarök Proofing: After thousands of years, during which (if the Doctor's correct about their location) every visible sign of the great city of London has gone to dust, the transmat platform is still intact and operational apart from a few humorous glitches.
- Redemption Equals Death: Vural is killed while helping the Doctor distract Styre.
- Rewarded as a Traitor Deserves: Styre wants to kill Vural as a punishment for betraying his friends, but they try to save him. Somewhat later, he dies anyway.
- Robot Antennae: Styre's robot has two pairs, on either side of its face, in an arrangement vaguely suggestive of an animal's whiskers.
- Sequel Episode: This serial happens immediately where the last one left off, and contains a lot of references to it, but no characters appear in both (except Doctor and companions, of course).
- Also to "The Time Warrior".
- Some Kind of Force Field: The force field Styre uses to prevent Sarah being rescued is completely invisible, but makes a buzzing noise when people interact with it. Its existence is established by Harry walking into it inadvertantly and then poking it a few times.Harry: I can't get in.
Sarah: There must be a force field.
- Spoiler Title: Completely ruins the story's sole cliffhanger. The writers were not happy.
- Torture Technician: Styre is a more resilient example.
- Weaksauce Weakness: Styre lists some of the human race's weaknesses and considers them Weaksauce Weaknesses compared to a Sontaran's strength.
- You Have Outlived Your Usefulness: Styre casually shoots Roth dead after deciding he's not necessary for the remaining experiments.