The one on drugs.
Continuing the series' general policy at this point of alternating history and science fiction-themed stories, we head to the planet Vortis, populated by the butterfly-like Menoptra and ant-like Zarbi. The latter have fallen under the influence of the Animus, a parasitic creature which is slowly enveloping the whole planet in webs.
While the camera lens gets a bit foggy from the sheer weirdness of this planet, the TARDIS crew are split up. The Doctor and Vicki try to keep themselves alive while dealing with the Animus, while Ian ends up in the underground home of the Optera. Barbara spends the serial in the company of a scouting party of Menoptra. Ultimately everyone converges on the lair of the Animus and (barely) manage to defeat it thanks to an untested weapon built by the Menoptra.
Novelized under the title Doctor Who and the Zarbi.
- Absentee Actor: Barbara is absent from the third episode in order to give Jacqueline Hill a holiday. She was not credited on the episode. She requested that the credits be amended for overseas sales, but this did not happen.
- Alien Sky: Vortis has multiple moons, drawn there by the Animus.
- All Webbed Up: Well, only their heads and upper bodies, but the Doctor and Vicky get this treatment.
- Anti-Villain: The Zarbi are actually being brainwashed by the Animus.
- Badass Adorable: The larva guns are deadly, but also very cute.
- Beneath the Earth: Ian meets pale, flightless descendants of the Menoptrans, who'd survived underground for generations since the Zarbi took over the surface of Vortis.
- Big Creepy-Crawlies: The Zarbi and the larva guns.
- Blinded by the Light: The Animus gives off quite a bright glow.
- Bug War: In this case, bugs versus bugs rather than bugs versus humans.
- Comically Missing the Point: Meta example. The BBC retains the late-Sixties Spanish dub of episode 6, wherein the Animus, whose voice is supposed to be soothing and hypnotic, sounds like a man talking into a staticky walkie-talkie.
- Continuity Nod: Barbara finally gets on the same page with Vicki about her adventures in Rome. The gold bracelet that Nero gave her plays a small part in the first two episodes.
- Conveniently Timed Distraction: Barbara and the Menoptera confront the Animus to save the Doctor and Vicki and kill it, but the Animus overpowers them with its light and blocks their attack. The Animus then gets briefly distracted when Ian and the Vrestins arrives from under the ground and Barbara quickly uses this opportunity to kill the Animus.
- Does This Remind You of Anything?: Vortis' natural ecosystem is compared to a single living organism. The Animus is constantly growing and sucking the life out of it. Its home base is called the Carcinome.
- Eldritch Abomination: The Animus qualifies both in terms of its powers and in terms of its appearance.
- Foreshadowing: Ian comments on ants eating through a house and being threatened by the ant-like Zarbi. In the next story, he is almost eaten by an ant colony.
- Handicapped Badass: Hrostar and Hlynia's wings were cut off when they were taken to the Crater of Needles. They still can kick Zarbi ovipositor.
- In-Camera Effects: The unique look of this serial was achieved by attaching a distorting lens to the camera.
- Insectoid Aliens: The four regular characters are the only humanoids in this story. For six episodes they spend time on a planet where every other living thing is an Insectoid Alien.
- Light Is Good: The Menoptra believe in gods of light, and convince the Optera that light is not bad. But when they see the light of the Animus, they have to remind themselves this creature is not good simply because it is light.
- Meaningful Name:
- Animus is a word meaning both a spirit, and a hatred or hostility.
- In the French slang verlan, which reverses syllables, Zarbi means "bizarre".
- Mind-Control Device: The Zarbi have a gold collar that they put around people's necks to control them, since the Animus can use gold to exert mental control over someone. Barbara is forced to leave the TARDIS in a trance-like state in episode one because she's wearing the gold bracelet that Nero gave her.
- Non-Mammal Mammaries: Averted; some of the Menoptra are female, but you'd never know it but for their voices.
- Obscured Special Effects: Scenes with the Zarbi are shot through a greased lens in an attempt to obscure how unconvincing the cheap ant costumes are. It doesn't work, mostly because they have a pair of hilariously human legs in trousers and shoes sticking out.
- Our Monsters Are Weird: Everything - the Menoptera, bee-butterfly humanoids with AcCENT upon the Wrong SylLABle speech that communicate partially through hand gestures; the Zarbi, bipedal giant ants that communicate through synthesised beeping sounds and have a larval stage with a long nose that it can fire like a gun; the Optera, grunting trogladytic creatures that speak in barely comprehensible metaphor; and the Animus, a spider-like Eldritch Abomination that communicates through web tunnels. The surreal look of the serial is the point here.
- People in Rubber Suits: The Zarbi: giant ants with two very, very humanoid legs.
- Puppeteer Parasite: The Animus.
- Starfish Language: The Zarbi.
- That Old-Time Prescription: Since she was from the twenty-fifth century, Vicki has schooling in many advanced degrees, including medicine; however, she has never heard of the drug that Barbara gives her for her headache: As-pir-in. She asks Barbara how she'd feel if Vicki prescribed leeches and blood-letting.