The one where the TARDIS Team plays Scooby-Doo.
The Doctor, with Ian and Barbara still in tow, arrives on the planet Dido some time in the 25th century. The planet is home to a crashed Earth spaceship, with two survivors on board: the paralysed Bennett and the young orphan Vicki. Both are fearful of a native monster called Koquillion, who has apparently killed the other members of the expedition.
Ian and Barbara soon run into Koquillion, and the Doctor explains that despite Koquillion's spiky exterior, the natives of the planet are extremely friendly. This one isn't, though, but before the Doctor can investigate, he and Ian are unceremoniously placed in a Death Trap. With a rabid beast lurking below.
While they escape, Barbara is hidden by Vicki, and tries to come up with a plan. Vicki keeps trying to reassure her new friend that a plan won't be necessary, since the rescue crew is due to arrive in a few days. But Barbara doesn't want to wait that long and goes off to find her friends, shooting the rabid beast in the process... only to discover that it was Vicki's tame pet, the only close friend the girl had left in her life.
The Doctor mediates between a very distraught Vicki and his humans, and decides to have a good long talk with Bennett. He finds out that Koquillion is in fact Bennett in disguise (he's not really paralysed, of course), and he has killed the other crew members in order to conceal a murder he committed earlier. He's also been killing the native Didonians, and believes he has in fact wiped them out — but a confrontation with two Didonians causes him to fall to his death from a high cliff. The Didonians have had enough of outsiders for a very long time, so they make sure the rescue team never arrives. It doesn't need to, since the Doctor misses Susan so much that he agrees on taking Vicki along.
- Adopt the Dog: The Doctor was already Neutral No Longer during "The Sensorites", but this story sees him inviting someone into his TARDIS.
- Artistic Licence Gun Safety: Vicki checks the load on her flare pistol, and readies it while pointing it directly at Barbara's face.
- Beard of Evil: Bennett.
- Booby Trap: Ian gets caught in one: walking also a narrow ledge, he gets trapped when two metal gates block the path in front of him and behind, following which spikes emerge from the wall to send him over the edge, where a monster lurks below. Given that the drop is just a few feet and the monster is a harmless sand beast, it seems that the peaceful Didonians rigged the trap as a practical joke.
- Bottle Episode: A rare example of a two-episode serial (the only one before now was "The Edge of Destruction", and the format would not be used again until "The Sontaran Experiment" a decade later) with only two credited guest actors. It was written by the departing script editor and all the incidental music was recycled from "The Daleks". This was because the serial's primary focus was as a vehicle to introduce Vicki.
- Call-Back: The crew discuss their problems with caves ("An Unearthly Child", "The Daleks", "Marco Polo").
- Continuity Nod:
- Reference is made to Susan's departure in the previous serial and at one point the Doctor calls for Susan, momentarily forgetting that she has gone.
- The Doctor, Ian and Barbara talk about how they met.
- Convenient Replacement Character: Immediately after leaving Susan on 22nd-century Earth, the Doctor comes across a sweet teenage orphan interested in adventuring.
- Death by Irony: Bennett tries to kill Barbara by pushing her off a high cliff while disguised as a Didonian in a ceremonial garb. When the real Didonians came out to confront Bennett, they indirectly cause him to fall off a high cliff and die.
- Death by Origin Story: Vicki's father was killed in a massacre just prior to the start of the story.
- Dirty Coward: Bennett, he's reluctant enough to get into a fight with an old man when he thinks he has the advantage, but when confronted by two young, fit and very very angry Didonian survivors he just backs away in terror.
- Disney Villain Death: Bennett is so shocked by the appearance of the people he thought he'd killed that he backs away over a cliff edge.
- Don't You Dare Pity Me!: See the page quote.
- Exposition Victim: The Doctor foolishly does this with Bennett and comes very close to getting killed. He defends himself well, though.
- Hands Off My Fluffy!: Barbara shoots Vicki's pet monster because she thinks it's attacking her.
- Heroic BSoD: At the start of the serial, the Doctor habitually asks Susan to help him with the TARDIS, only then realising that his beloved grand-daughter really has left. The awkward few seconds of the Doctor helplessly standing in silence really sell it.
- Humans Are the Real Monsters: Bennett is the true main antagonist of the serial. However he is clearly not any sort of standard
- I'm Standing Right Here: The Doctor comes out of the TARDIS to tell Ian and Barbara that he can hear what they're saying about him right outside.
- Literal Cliffhanger: The story ends with the TARDIS materialising at the edge of a cliff, only to tumble over.
- Meta Twist: By this point fans had gotten used to the show's not so realistic costumes for aliens, so the reveal that Koquillion is really just a human in a costume and the Didonians actually look like normal humans, is a pretty good surprise. The Koquillion costume is actually a bit better looking than some other alien costumes of the era that are meant to be real monsters, not least Vicki's pet "Sandy".
- Never One Murder: Bennett murdered a single crewman, to cover up his crime he killed the rest of the crew and then attempted genocide against the peaceful Didonians. He likewise tries to kill the Doctor, Ian and Barbara when they arrive.
- Obfuscating Disability: Bennett.
- People in Rubber Suits: Interestingly enough, this trope to conceal the twist. Fans were used to rubber suit aliens, so it actually came as a surprise that the villain really was a human wearing a costume, not a real alien. And it was more convincing than many of the "real" alien costumes of the era.
- Properly Paranoid: Vicki only not really.
- Recycled Soundtrack: To save money, the score is reused from "The Daleks", which Christopher Barry had partially directed. He selected pieces from Episodes One, and Four to Seven of that serial.
- "Scooby-Doo" Hoax: The spiky alien look is just ceremonial garb.
- Shout-Out: The "Sidney Wilson" credit was a joint shout out to the two BBC executives responsible for Doctor Who, Donald Wilson, the Head of Series and Serials, and Sydney Newman, the Head of Drama.
- The Sociopath: Bennett is one of the most disturbed examples in the shows whole history, content to commit mass murder to the scale of outright genocide to save himself. What's most scary is just how casual he is about what he's done.
- Space Clothes
- Unbuilt Trope: The story is a deconstruction of the show's whole modus operandi. The TARDIS team land on a planet where a young woman, Vicki, in a crashed spaceship is waiting to be saved by a rescue vessel, while also being kept prisoner by an alien named Koquillion who has killed the rest of the crew. Barbara murders the young woman's pet monster, assuming it was trying to eat her, and the Doctor talks to the other survivor of the crash about dealing with Koquillion, after which he points out that they have nothing to gain from doing that, as the rescue ship is already coming. This culminates in Vicki telling them all that they have no right to go around landing on other people's planets assuming they know exactly what to do when they aren't living there and have no real idea what's going on or if their attempts to fix it are just making everything worse, that she hates them, and that they should all just leave. It is all fixed in the end, but only because the Doctor had already visited the planet previous to the story, listens to Vicki's point of view while still questioning things she's too entrenched in her own ways to question herself, and because the natives of the planet eventually take matters into their own hands and deal with Koquillion. Furthermore, Koquillion isn't an alien at all; he is the other survivor who invented a phony alien plot to cover up his murder of the rest of the crew.
- You're Insane!: The Doctor tells Bennett this during their confrontation.
- Zeerust: Apparently tape recorders will still be in use in the 25th century.