A mysterious force pulls the TARDIS off course, stranding The Doctor, Ian, Barbara, and Vicki on the planet Vortis, where a war is raging between the moth-like Menoptra and the Zarbi, who look like giant ants and produce horrible siren-like noises. The Tardis has no power. Ian and The Doctor go out onto the barren surface and strange things begin to happen.
Barbara is pulled out of the TARDIS in a daze by something invisible. Ian is caught in a net. The Doctor runs back to the TARDIS to get something to free him and finds that the time-machine has disappeared. He manages to free Ian and they discover tracks indicating that the TARDIS has been dragged away, which they follow.
The Menoptra are holding Barbara, but she escapes. The two men find a chrysalis and are captured by the Zarbi, as well as Vicki. The Doctor communicates with them, but they do not believe he is peaceful, insisting that he is in league with the Menoptra. They attack the TARDIS and it strikes back, whereupon The Doctor is able to immobilize the Zarbi.
Barbara is trying to help the Menoptra as she believes the Zarbi are the aggressors in the war. The Menoptra, which have almost been wiped out, are captured and taken, along with Barbara, to the Needles work camp. The Doctor, stalling for time, pretends to help the Zarbi and is taken to The Animus, which is a Central Intelligence in the shape of a huge spider-like creature. They manage to kill it, saving the Menoptra and the planet.
The entire story was believed lost, but it was found and restored in the 1970s. One might prefer that something less silly had been saved, like Marco Polo. I’m afraid the humans in giant insect costumes never approach believability, especially the winged Menoptra, though a choreographer named Roslyn de Winter created balletic movements and stilted speech to give them a certain fey charm. But one must remember that Doctor Who was considered a children’s program, and I suspect that children found them charming enough to suspend disbelief.