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 episode sees him inviting someone into his TARDIS.
- Artistic License 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.
- 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.
- Getting Crap Past the Radar: Some of Ian's mocking mispronunciations of "Koquillion" verge on this.
- 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
- You're Insane!: The Doctor tells Bennett this during their confrontation.
- Zeerust: Apparently tape recorders will still be in use in the 25th century.