The one where the Doctor defeats some aliens by shouting.
Time for another space adventure as the TARDIS gang appear on an Earth spaceship orbiting the planet Sense-Sphere in the 28th century. The crew of the spaceship all appear to be in comas, which it turns out have been induced telepathically by the planet's denizens, the Sensorites. The Sensorites then steal the TARDIS's lock mechanism, trapping the travellers on the space ship in orbit around the planet.
It turns out that the Sensorites are more scared of the humans than the humans are of them, because they're a very sensitive telepathic race. Luckily, Susan reveals that she's exceptionally gifted at telepathy, and she makes peace with them. The Doctor, Susan and Ian decide to visit the planet and try diplomacy, while Barbara stays behind on the spaceship.
It transpires that a fatal disease is spreading like wildfire among the Sensorites, and immediately, Ian comes down with it as well. The Doctor realises that the planet's water supply is tainted with deadly nightshade. He investigates despite the interference of the City Administrator (and a lot of political intrigue), and eventually manages to convince the Sensorites that he's safe to trust. However, by that time, the entire Sensorite government is in uproar because of the scheming City Administrator realising that he can fool everyone simply by switching clothes. To make matters worse, the Doctor actually suggests that the City Administrator be promoted. Luckily, Barbara rejoins the plot on the planet, immediately realises what's going on and starts setting everything straight by establishing a psychic link with Susan (using Sensorite telepathy technology).
Going back into the sewers with Ian, the Doctor eventually discovers the deranged survivors of a previous Earth expedition, who have been poisoning the water supply, believing that they're at war with the Sensorites. The Apocalypse Now enactment group is swiftly arrested, the Doctor whips up a cure and all is well. The humans of the spaceship decide to head home — and when Ian comments that they at least can steer their ship, the Doctor throws a bit of a hissy fit and tells Ian that he and Barbara will be unceremoniously dropped off at the next destination for insulting his piloting skills.
Due to the similar design of the Sensorites to the Ood in the new series, in "Planet of the Ood", Sense-Sphere is shown to be in the same system as Oodsphere.
- Absentee Actor: Barbara is absent from episodes 4 and 5 in order to allow Jacqueline Hill a holiday.
- Actor Allusion: Stephen Dartnell previously played a character similar to John in "BBC Sunday Night Theatre".
- The Alleged Car: "This old ship of mine seems to be an aimless thing."
- And Your Reward Is Clothes: The Sensorites give the Doctor a new cloak when is coat is destroyed in the acqueduct.
- Badass Boast: The Sensorites have decided that letting the crew leave is too dangerous, and have taken a vital component of the TARDIS. They act like they're totally in control, until:Sensorite: "You're in no position to threaten us."The Doctor: "I don't make threats, but I do keep promises. And I promise you I'll cause you more trouble than you've bargained for if you don't return my property."[Sensorites trade looks, then one responds] "We must... decide what we shall do."
- It's also a good look into the First Doctor's character, there. He's a crotchety old man and does more than his share of yelling, but then you find his bark is worse than his bite... and then you find out what he looks like when he's actually ready to bite. When he stops being hammy and starts being serious, you wanna listen.
- Bald of Evil: Inverted: the "bad" Sensorite is the one with most hair on the top of his head.
- Character Development: The Doctor has another important development in this serial - halfway through, he has the opportunity to simply leave for the first time, but chooses instead to stay and help the Sensorites.
- Continuity Nod: One of the first clear signs of continuity in the series is that near the beginning, the Doctor and companions mention nearly all of their previous adventures together and how they've developed during the journey. It makes sense for an episode near the end of the first season.
- Counting to Three: Well, five.
- Disproportionate Retribution: At the end of the episode, the Doctor throws a huff and decides to dump Ian and Barbara at the next stop after Ian makes a joke about the Doctor's ability to fly the TARDIS.
- Ditto Aliens: Bizarrely, invoked by the Sensorites on themselves. They themselves cannot effectively tell the differences between each other without first having become familiar with them, and rely on sashes and other decorative garb to identify important individuals. This allows the City Administrator to steal the Second Elder's identity before and after killing him. The City Administrator at least takes the effort to avoid getting close to the First Elder or any other Sensorite who regularly interacted with the Second Elder and expositions that the disguise only fools those Sensorites who only see the Second Elder at a distance or in passing.
- Dramatic Irony: The City Administrator spends nearly all of his screentime trying to kill the Doctor et al., and yet after killing the Second Elder and indirectly framing the Doctor for it he gets a recommendation for a promotion from the Doctor himself for his trustworthiness. Ian even comments on this.
- Failed a Spot Check: John amazingly fails to see Barbara and Susan to his immediate left. Then he kindly insists on looking in the opposite direction of them on his way out.
- Humans Are the Real Monsters: This is a zigzag as while humans were guilty of the poisoning, they were actually driven mad by the Sense-Sphere, so their villain status is left ambiguous, while the speciesist City Administrator was, in the end, the true villain (though as far as Sensorites go, he was an exception). The First Elder even notes that Sensorites have much to learn from "the people of Earth", because he mistook Susan for a human. She points out that she is not from Earth but her home planet is very much like Earth.
- Hypocritical Humor: This gem makes both Ian and Barbara laugh.Doctor: I learned not to meddle in other people's affairs years ago... Now, now, now, don't be absurd. There's not an ounce of curio—curiosity in me, my dear boy. Tell me, why are you in danger?
- I Have Your Wife: The City Administrator tries this on the Second Elder.
- Indy Ploy: The City Administrator's general plan is "kill the humans". He never really works out the details.
- Idiot Ball: One of the humans points out to one of the Sensorites that Sensorites are all face-blind and cannot tell each other apart besides their clothing, to which the Sensorite responds "I hadn't thought of that" despite this being part of his biology. This sets off his plot to impersonate another Sensorite by stealing his clothes.
- Knight Templar:The City Administrator,who apparently wants to kill all humans for no other reason than speciesism.
- Leaning on the Fourth Wall: There's a part where the companions all talk about how they've all changed since they set out with the Doctor, and he agrees "It all started as a mild curiosity in a junkyard, and now it's turned out to be quite a spirit of adventure."
- Magic Antidote
- Mind Rape: The Sensorites often torture the humans with telepathy. Most humans aboard the ship are worn-down and stressed out from the abuse, but John in particular is mostly nonfunctional as a result of being constantly dripfed psychic terror for months, his hair has gone white from stress and his fiancée mourns his old personality as if he was dead. The Doctor is not pleased. He eventually persuades the Sensorites to restore his mind, but they note that even after the treatment he'll bear permanent psychological scars.
- Neutral No Longer: The moment when the Doctor decides to investigate the aqueduct to find the source of the atropine is the first time that he does something out of altruism without being forced into it, a major point in his transformation from untrustworthy vagabond mad scientist to the hero we know and love.
- Never Split the Party: Ian is aware about it at this point, but acknowledges that they have no choice in the matter.
- Nightmare Face: The hideous alien face of a Sensorite pressed up against the glass of a spaceship in the first Cliffhanger.
- Noodle Incident: That hilarious quarrel with Henry VIII that The Doctor recalls (that apparently took place when it was just him and Susan travelling).
- Not Quite Dead: The three members from the ship who originally encountered the Sensorites turned out to survive the crash.
- Offscreen Karma: The City Administrator got banished offscreen for his actions.
- People in Rubber Suits: The Sensorites are people in velour suits, with visible zips.
- Prematurely Grey-Haired: John's hair has gone white as a result of his being subjected to psychic Mind Rape for months.
- Pstandard Psychic Pstance: Susan whenever using telepathy.
- Psychic Powers: Susan reveals hers. The Doctor also claims that he can read his companions' minds, but later notes that Susan is significantly more gifted in that regard.
- Psychic Static: Barbara and Susan think together "I defy you!" to protect John from the Sensorites.
- Smart People Wear Glasses: The Doctor sports both regular glasses and a High-Class Glass throughout this episode.
- Talking to Themself: What it looks like John is doing in episodes 1 and 2. He's actually communicating with the Sensorites trying to get him to kill Susan and Barbara.
- They Should Have Sent A Poet: Susan's description of Gallifrey is gorgeous, and would be referenced by the Eighth and Tenth Doctors later on.
- Unbuilt Trope: This is the first time we see a human space empire. The humans coming to the Sense-Sphere is treated as a bad thing, as a previous mission is hiding on the planet and poisoning the inhabitants in an attempt to steal their minerals. The Sensorites themselves are portrayed sympathetically, trying to protect themselves while the main villain is the scheming City Administrator. The story could also be seen as a slight Deconstructive Parody of Ditto Aliens, as the Sensorites can only recognise each other by their clothes, enabling the villain to take the place of one simply by killing them and stealing their clothes. Even so he avoids getting close to anybody who knew this Sensorite well and points out his disguise only works on those who saw the Sensorite at a distance or in passing.
- Unobtainium: Averted. Molybdenum is, in fact, an actual element often used in steel alloys.
- Water Source Tampering: The Sensorites are being affected by a disease that the good guys later trace to a human expedition, maddened by the Sense-Sphere, poisoning their water supply.
- Weaksauce Weakness: The Sensorites are terrified of the dark (Due to their eyes working like the OPPOSITE of a cat's eyes). and physically pained by raised human voices.
- What the Hell, Hero?: The Doctor chews Susan out for acting on her own and trying to go with the Sensorites.
- Wrong Genre Savvy: The City Administrator's final plan to kill the Doctor and his companions was when they went to investigate what was the source of the poisoned water, knowing that there was something down in the caves killing any Sensorites who went down there, makes sure they are given inaccurate maps and their weapons are sabotaged before entering, certain that whatever is down there will kill them. This likely would have worked, except what was hiding down there turned to be humans.
- Xanatos Speed Chess: Again, the Administrator.
- Yellowface: The Sensorites were based off of Chinese Communists.