The one with fish people. And the other one where the Doctor flips his lid in anger.
The TARDIS is on its way to Gallifrey to return Romana home, a task the Doctor has been putting off (and Romana has been letting him) for over a season, when it falls into a Negative Space Wedgie. The result is apparently only a momentary disturbance, but when they arrive at their destination, it isn't Gallifrey. They're in the right place, and the right time, but — they eventually discover — the wrong universe. (For ease of discussion, they subsequently dub the two universes "Normal Space" and "Exo-Space", or "N-Space" and "E-Space" for short.)
Meanwhile, the inhabitants of the planet that isn't Gallifrey have problems of their own. Years ago, a starliner from the planet Terradon made a crash landing on the planet, where they were attacked by the local lifeforms. The survivors of the attack set about trying to repair the starliner and return to Terradon, a task at which their descendents are still working. And now the season of Mistfall is approaching, when all take shelter within the hull of the starliner to avoid the mysterious dangers lurking in the fog. But there are dangerous mysteries inside the starliner as well.
At the end of the story, Adric, a young boy from the starliner, stows away in the TARDIS.
- Berserk Button: Dexeter and the Deciders make the extremely huge mistake of pushing the Fourth Doctor's, by first allowing the death of the Marshchild and then mumbling excuses about it - it leads, naturally, to the Doctor losing all of his shit at them:
- "NOT AN ALIBI, DECIDERS!"
- Also, don't steal Adric's badge. His first scene has him putting Keara in a stranglehold for trying to do so.Keara: So the boy can count. Give it to me! (She makes a grab for Adric's badge.)Adric: Keep your hands off it! (He grabs Keara's arm and twists it behind her back.)
- Can't Argue With Time Lords: Romana is summoned back to Gallifrey. The Doctor's casual invokation of this trope almost reduces her to tears.
- Continuity Nod: The Doctor briefly mentions Leela, K-9 Mk I and the Key to Time. He even hopes that both K9s meet each other.
- Death by Origin Story: Varsh, whose death at the hands of the Marshmen means Adric no longer has any ties among his people.
- Death of a Child: The Marshchild dies (and is, in fact, the only Marshman to perish in the story). Tylos and Varsh also arguably count, as both are teenagers who die during the Marshmen's rampage aboard the Starliner in the final episode.
- Despair Event Horizon: Romana is clearly fighting not to cry at the thought of being forced back to Gallifrey with no say in the matter.
- Establishing Character Moment: Adric's first scene has him trying to join the gang which his brother, Varsh, has formed. This scene includes a close-up of Adric's badge for mathematical excellence, as well as establishing that he has a rather high opinion of himself. "Of course I'm better than you. I'm an Elite."
- Everyone Is a Tomato: The people on the Starliner are indigenous Alzarian lifeforms and not descendants of the original crew and passengers.
- Fog of Doom: Subverted; K9 reveals that it's non-toxic.
- Forgotten Fallen Friend: Adric only mentions his brother Varsh's death once after this story and seems to suffer no emotional repercussions from it, much to the chagrin of Matthew Waterhouse, who had lost his own older brother in real life.
- Friend to All Children: As usual, the Doctor.
- Generation Ship: One that isn't actually going anywhere.
- Healing Factor: It's a bit of a plot point in this story that all of Adric's people can heal faster than human normal. You'd think this would be handy for a companion, but it turns up once in Adric's stories and then never again.
- "I Know You're in There Somewhere" Fight: The Doctor to Romana.
- Infant Immortality: Sadly averted with the Marshchild.
- In Harm's Way: Romana really doesn't want to stay on Gallifrey again.
- Negative Space Wedgie: Here, a "Charged Vacuum Emboitment."
- Not-So-Omniscient Council of Bickering: The leaders of the Alzarian community are called the Deciders. In a Running Gag, they can't come to an actual decision without being shoved into it.
- People in Rubber Suits: The Marshmen
- Plug 'n' Play Technology: The image translator.
- Propaganda Machine: Varsh and his friends are convinced that Mistfall is this.
- Stock Episode Titles: 45 uses.
- Tap on the Head: The Doctor himself gets this treatment — keeping him from calming the alien child.
- The Team Wannabe: Adric is first seen in the Outlers' cave, trying to persuade them to let him join them. Since he's an Elite, they're somewhat reluctant to accept him.
- Tomato in the Mirror: The revelation that the Alzarians are really just evolved Marshmen, who replaced the original Terradon crash survivors after the survivors all died off or were killed.
- Two Guys and a Girl: Varsh, Tylos and Keara.
- Ungrateful Bastard: When the young Tylos goes back against Varsh's warnings and saves one of the crew from a Marshman. The man pauses as he sees Tylos is being attacked, and promptly leaves him to die in his place.
- Up the Real Rabbit Hole: Of course our universe is the Normal one.
- Wham Line: Nefred's last words.
- Nefred: We cannot return to Terradon.Login: Why?Nefred: Because we have never been there.
- "You!" Exclamation: "DEXETER!!!!"