[[caption-width-right:350:The First Doctor, seen establishing a trend that crops back up later down the road: [[PurelyAestheticGlasses The Brainy Specs]].[[note]]Yes, yes, it's actually a monocle, but...[[/note]]]]
->''It all started out as a mild curiosity in the junkyard and now it's turned out to be quite a great spirit of adventure.''
-->-- '''The Doctor'''
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 realizes 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 realizing 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 realizes 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 ''Film/ApocalypseNow'' 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 [[Recap/DoctorWhoS30E3PlanetOfTheOod "Planet of the Ood"]], [[AscendedFanon Sense-Sphere is shown to be in the same system as Oodsphere.]]
* ActingForTwo: Several of the Sensorite actors play what are clearly intended to be different characters in different episodes.
* AllThereInTheManual: According to the novelisation, the humans are part of an extremist group called INNER.
* TheAllegedCar: "This old ship of mine seems to be an aimless thing."
* BaldOfEvil: Inverted: the "bad" Sensorite is the one with most hair on the top of his head.
* ContinuityNod: 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.
* CountingToThree: Well, five.
* DisproportionateRetribution: 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.
* DittoAliens: 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 relies on sashes and other decorative garb to identify important individuals. It 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 Sensorites 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.
* DramaticIrony: 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 ''[[ItMakesSenseInContext from the Doctor himself]]'' for his trustworthiness. Ian even [[LampshadeHanging comments]] on this.
* FailedASpotCheck: 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.
* HumansAreTheRealMonsters: This is a zigzag as while humans were guilty of the poisoning, they were actually driven mad by the sensosphere, so their vilainy 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.
* HypocriticalHumor: 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?
* IHaveYourWife: The City Administrator tries this on the Second Elder.
* IndyPloy: The City Administrator's general plan is "kill the humans". He never really works out the details.
* KnightTemplar:The City Administrator,who apparently wants to kill all humans for no other reason than speciesism.
* NeutralNoLonger: 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.
* NeverSplitTheParty: Ian is aware about it at this point, but acknowledges that they have no choice in the matter.
* NoodleIncident: That ''hilarious'' quarrel with Henry VIII that The Doctor recalls (that apparently took place when it was just him and Susan travelling).
* PstandardPsychicPstance: Susan whenever using telepathy.
* PsychicPowers: 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.
* PsychicStatic: Barbara and Susan think together "I defy you!" to protect John from the Sensorites.
* SmartPeopleWearGlasses: The Doctor sports both regular glasses and a HighClassGlass throughout this episode.
* TalkingToThemself: What it looks like John is doing in episodes 1 and 2. He's actually communicating with the Sensorites [[MindControl trying to get him to kill Susan and Barbara]].
* TheyShouldHaveSentAPoet: Susan's description of Gallifrey is gorgeous, and would be referenced by the Eighth and Tenth Doctors later on.
* WaterSourceTampering: The Sensorites are being affected by a disease that the good guys later trace to a human expedition, maddened by the sensosphere, poisoning their water supply.
* WeaksauceWeakness: 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.
* WhatTheHellHero: The Doctor chews Susan out for acting on her own and trying to go with the Sensorites.
* WrongGenreSavvy: 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.
* XanatosSpeedChess: Again, the Administrator.