"You and I are scientists, Professor. We buy our privilege to experiment at the cost of total responsibility."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.
— The Doctor
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- The X of Y
- Thrown Out the Airlock: Luckily Averted at the last second.