The TARDIS finds itself being pulled towards a planet's gravitational force rather mysteriously. Upon settling down on the planet surface,Barbara and Vicki find themselves both under the control of a mysterious power. While investigating for themselves,the Doctor and Ian discover a desolate world with acid pools and where Ian's gold watch disappears out of thin air. The doctor concludes that the planet is Vorbis,a world that he has some familiarity with. Barbara for her part finds herself among the peaceful Butterfly-like Menoptra. They are ensnared in a war with the Zarbi,large ants who have overtaken the moon,driving the Menoptra to the planets moon because of the Animus,a malevolent entity that has engulfed the planet in one life consuming web of it's own creation.
The intention of the Animus is to try to use the Doctors knowledge,through it's control device within what it calls the Carsenome,to influence the doctor in the same way they have the Zarbi and Barbara-through the use of their ability to control biological energy through gold alloy. The doctor,for his part however is able to learn their method of control and counteract it. He's even able to use one of the Animus's devices to take control over one of their Zarbi slaves to escape imprisonment. Once out from under the Animus's influence the doctor is able to manipulate the Animus's energy source at the very moment it's about to absorb the planet and renew Vorbis to the fertile environment it once was.
One of the most important elements of this story is the visualizations. While the sets and costumes are extremely obvious even by this era of Doctor Who's standards the use of lighting,filter and the almost experimental theater style of acting used in particular by the actors portraying the Menoptra do more to tell this story then the actual plot. References to enforced evolution,in the place of the Menoptra and their flightless cousins the Optera,as well as alchemy are key elements to this story. In truth,it is a very slow story in parts. And would've been brilliant (and equally as effective) if told in three or four parts rather than six. Still,even by Doctor Who standards,this is a particularly mind bending story.