Leela: The Evil One!The TARDIS arrives on a planet where the primitive native tribe known as the Sevateem immediately identify him as "The Evil One", and worship a God called Xoanon. They've got a point, seeing as there's a giant invisible floating Tom Baker face terrorising them, and there's an even bigger Tom Baker face carved into the nearby mountains. A young female warrior called Leela, meanwhile, has some issues with the existence of Xoanon, and faces a trial for heresy. Her father decides to take the trial instead, and promptly dies just a couple of minutes into the serial.Leela, now Conveniently an Orphan, is forced to leave the tribe and runs into the Doctor (who deduces that the TARDIS probably hasn't landed him in Hyde Park after all). After both are captured by the Sevateem again a few times over, she shows the Doctor the image of the Evil One — his own face carved into the side of a mountain. The Doctor tries to convey that the tribe's "sacred relics" are bits of space suits and defunct spaceship parts, and that the Sevateem are just a group of humans descended from a crashed explorer ship. Leela won't have it, and the local Large Ham priest, Neeva, suffers a bit of a religious breakdown because of it.The Doctor investigates further (into the giant statue's mouth, to be exact), and discovers that Xoanon is really a ship's supercomputer. A long time ago, he tried to repair it when it went haywire, by linking it to his own brain and connecting it with the relevant data in there. (The official 70's novelisation states that he did it during "Robot", when he popped into the TARDIS for a while.) But the computer wasn't broken at all — it had achieved sentience, and its consciousness had just been born. The newborn computer-creature went "schizophrenic" due to being linked to the Doctor's mind, and developed a second personality: that of the Doctor. With the two personalities fighting for dominance, the ship crashed, and Xoanon decided to trap its survivors in a large-scale eugenics experiment. The Survey Team, which explored the planet, were modified to show strength and courage and became the Sevateem; the Tech Team became the (now legendary) Tesh, granted cold reason and psychic ability by Xoanon. Generations later, both parties have now fallen to Cargo Cult superstition, allowing Xoanon to rule as a God for both.The Doctor, Leela and Neeva (who very quickly catches on, surprises the Doctor with his intelligence and makes a Heel–Face Turn) manage to access Xoanon's memory banks, after running the gauntlet of traps and mazes set both by the Tesh and Xoanon himself. They erase the Doctor part of Xoanon's multiple personalities, leaving the computer sane once more. Leela is promptly elected as the leader of both tribes. Much like the Doctor in the last episode, she's not remotely interested in becoming president of anything, and she runs off to become a companion instead. Despite the Doctor's protests.
The Doctor: Well, nobody's perfect, but that's overstating it a little.
The Doctor: Well, nobody's perfect, but that's overstating it a little.
- Action Girl/Nubile Savage: Leela. Oh yes.
- Aerith and Bob: The Sevateem people are called Leela, Neeva, Andor, Calib and... Tomas?
- A.I. Is a Crapshoot: Played with — the computer is mad, but it's entirely the Doctor's fault, and it ends with his restoring its sanity rather than blowing it up.
- All There in the Manual: Terrance Dicks' novelisation of  states that the Doctor's first trip to the planet occurred during his trip in the TARDIS in that episode.
- The backstory of how and when the Fourth Doctor met Xoanon is discussed in the novelisation.
- The Doctor believes he was destined to return to the planet rather than return to join Sarah.
- Attack! Attack! Attack!: The Sevateem under both Andor and Calib.
- Big "NO!": Xoanon even makes it a Cluster No Bomb.
- Brainwashed: Xoanon can make the Sevateem try and kill the Doctor with seemingly no more than a thought.
- Breaking the Fourth Wall: Since the Doctor starts the story without a companion, Tom Baker awkwardly does some schtick directly to the camera instead.
- Cargo Cult: Both the Tesh and the Sevateem, to different extents. Played with a bit in that they're worshiping their own technology, just from a different point in time.
- Deus Est Machina
- Don't Touch It, You Idiot!: The last words we hear are the Doctor shouting at Leela not to touch that button after she's slipped inside the TARDIS...
- Eats Babies: Spoofed:The Doctor: Would you like a jelly baby?
Leela: It's true, then. They say the Evil One eats babies.
- Evilutionary Biologist: All of the characters are victims of a eugenics experiment being run by Xoanon. By and large, they bear him no hard feelings for it afterwards.
- The Exile: The story begins with Leela being banished.
- Heel–Face Turn: Neeva and Calib.
- Heroic Sacrifice: Neeva, not entirely intentionally.
- Leela's father offers up his life to save his daughter.
- Hold Your Hippogriffs: The Doctor coins the phrase 'As dead as a Dalek'.
- Human Shield: Done by the Doctor of all people. When confronted by the savage Sevateem tribe, the Doctor threatens to kill one of them "with this deadly jelly baby" if they don't back off. The Sevateem surprise the Doctor by responding, "kill him, then." Determined not to be upstaged, the Doctor simply eats the jelly baby instead. "I don't take orders from anyone. Take Me to Your Leader."
- Invisible Monsters: Deconstructed. The Doctor works out that they can't see, since light would just pass through any eyes they had, and must rely on other senses. This lets him distract them with an alarm clock long enough for him and Leela to get to safety.
- Jumped at the Call: Leela.
- Just One Second Out of Sync: How the Wall works.
- Lost Colony
- Mondegreen: After a few generations of tale-telling, Survey Team becomes Sevateem and Technicians becomes Tesh.
- Monster-Shaped Mountain: The giant carving of the "Evil One".
- Nerves of Steel: Leela's "fight or flight" response doesn't have a "flight" setting.
- Nice Job Breaking It, Hero
- Noble Savage: All of the Sevateem, but Leela in particular.
- Pay Evil unto Evil: The Doctor kicking a vicious Horda onto the back of a Sevateem man who smacked Leela in the face.
- Piranha Problem: The Test of the Horda is a land-dwelling version.
- Please Kill Me If It Satisfies You: After Xoanon is healed, it makes this offer to the humans via the Doctor.
- Romanticism Versus Enlightenment: The Sevateem versus the Tesh.
- Satan Is Good: From the Sevateem's point of view. Also, Leela is pretty quick to team up with the Doctor when she still thinks he's the "Evil One".
- Shoot the Rope: The Sevateem give the Doctor a Trial By Ordeal which consists of this.
- Spoiler Cover: The video and DVD covers feature the gigantic stone carving of the eponymous face. However, the reveal that the face was that of the Doctor was the dramatic cliffhanger of one episode.
- Strapped to an Operating Table: Leela and the Doctor, by the Tesh.
- Stripperiffic: The Sevateem dress sense, male and female.
- Take Me to Your Leader
- Tempting FateDoctor: If they're preparing for a battle, they're hardly likely to send men on patrol on the off-chance that you might come back. (an arrow hits the tree inches from his face) On the other hand, I could be wrong about that.
- Thicker Than Water: Leela's father tries to take the test for her. He dies.
- The X of Y
- Walking Shirtless Scene: All the male Sevateem.