Apart from rubbing our legs together like some sort of grasshopper, I doubt if we can get on speaking terms with them.
— The Doctor ponders the difficulties of communication.
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 get 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.
- 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.
- 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.
- Continuity Nod: Barbara finally get 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.
- Eldritch Abomination: The Animus qualifies both in terms of its powers and in terms of its appearance. In the Expanded Universe, it's actually named as one of H.P. Lovecraft's Old Ones, Lloigor...
- 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 humans (well, three humans, one humanoid) in this story. For six episodes they spend time on a planet where every other living thing is an Insectoid Aliens.
- Light is Good / Light Is Not 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.
- 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.
- People in Rubber Suits: really obvious for this one.
- Puppeteer Parasite
- Starfish Language: The Zarbi.