The one with a lot of trippy video effects.
Investigating a Distress Call, the Doctor and Sarah land on Zeta Minor, where they discover the remains of a geological expedition from Morestra. Only the leader, Professor Sorensen, remains alive, the others all killed by an unseen beast. A Morestran military team has also arrived to investigate, and the "beast" is discovered to be a creature from an Antimatter universe. A pit on the planet acts as a gateway between the universes of Matter and Antimatter, and the creature has been taking revenge for Sorensen's removal of Antimatter samples from around the pit. As usual, the Doctor and Sarah get held responsible for the deaths.
The Morestrans attempt to leave, but the Antimatter on board drags their ship backwards, while Sorensen himself becomes infected with the Antimatter and begins to turn into an Anti-man, capable of draining the life from others. The Morestran commander, Salamar becomes increasingly insane and attacks Sorensen with a radiation source, but only succeeds in multiplying him, causing the ship to be overrun with deadly Antimatter creatures.
The Doctor captures Sorensen and takes him and the samples back to the pit in the TARDIS and throws them in. It is revealed that he has made a bargain to this effect with the Antimatter creature, and Sorensen is returned unharmed and the Morestrans allowed to leave.
- Actor Allusion: Prentis Hancock then notable for playing Controller Paul Morrow from Space: 1999 appears as, err, Controller Salamar.
- Antimatter: Sorenson wants to use it as an energy source. Interestingly, it doesn't annihilate with matter on contact, but the Doctor still fears it will.
- Batman Grabs a Gun: The Doctor has previously shown an intense dislike of guns, but he uses one in this episode. It was a stun gun, but still...
- Chekhov's Gun: The little box of antimatter the Doctor took.
- Chromosome Casting: Sarah is the only female character in the story.
- The Corpse Stops Here: The Doctor and Sarah Jane are found twice near a dead body. Of course, they are the only suspects for most of the serial.
- Death Is the Only Option: The Doctor gently explains to Professor Sorenson that the experiment has gone wrong, The Professor is infected with antimatter, and sacrificing himself is the only way to save everyone else. Averted, when circumstances dictate that the Doctor Take a Third Option.
- Deflector Shields: Used to keep the antimatter creature out. It even has light blue ripples.
- Distress Call: At the start of the adventure, the TARDIS picks up the Sorenson expedition's distress call.
- Easily Forgiven: Sorenson's greed caused the deaths of at least ten people, but when he's no longer possessed, everything is fine.
- Eldritch Location: Zeta Minor, where strange beings lurked and tried to prevent catastrophe caused by removing material from the place.
- Energy Being: The antimatter creature.
- Famous Last Words: Before going after Sorensen, the Doctor (very nearly) quotes Lawrence Oates's words before walking into an Antarctic blizzard: "I'm going out, and I may be some time." He makes it back.
- Fanservice: Sarah Jane wears a form-fitting bodice and skintight pants. A mild example of this trope compared to Leela's later costumes, but probably the most provocative outfit of her Doctor Who tenure.
- Homage: The anti-matter creature is one to The Forbidden Planet's Id monster, which is itself a version of the monster Caliban from The Tempest. Meanwhile, the nature of the planet comes from The Strange Case of Dr. Jekyll and Mr. Hyde.
- Lock Down: Averted due to the Intangible Man nature of the Antimatter monsters.
- Non-Indicative Title: The planet isn't really evil, just protective of its antimatter.
- People in Rubber Suits: The whole rationale behind the Jekyll-and-Hyde planet, the antimatter creature, and Sorenson's transformation was to avoid this.
- Pet the Dog: Even though Sorenson was uncooperative the whole time and contaminated with antimatter to boot, the antimatter creature let him live.
- Puppeteer Parasite: Contamination with antimatter makes Sorenson behave like the antimatter creature originally did.
- Glowing Eyes Of Doom
- Scenery Porn: Zeta Minor's forest set.
- Shout-Out to Shakespeare:
The Doctor: "I met Shakespeare, you know. Charming fellow, dreadful actor".Sarah: "Maybe that's why he took up writing".The Doctor: "Very probably".
- The Doctor quotes from Romeo and Juliet ("Night's candles are burned out and jocund day stands tiptoe on the misty mountain top. Or something like that") and Hamlet ("That is the question"), and says that he met the Bard once.
- Thrown Out the Airlock: Luckily Averted at the last second.
- Video Inside, Film Outside: The interior scenes were videotaped in the studio and exterior scenes on the alien planet were filmed on location the location in question being another studio, namely Ealing Studios, which required the use of film as opposed to videotape at the time.
- Whole Plot Reference: To Forbidden Planet, which itself was a whole-plot reference to The Tempest. It also uses elements from Universal's Jekyll & Hyde.
- Yellowface: According to the actor, the captain was supposed to look Chinese and had heavy latex prosthetics applied to create this effect. However, the makeup 'didn't go with [his] face', with the result that he doesn't even look like they were going for this, let alone look like an East Asian.
- The X of Y