The Zarbi

A strange power drain forces the TARDIS to materialise on the craggy, forbidding world of Vortis. While The Doctor and Ian Chesterton leave the TARDIS, to investigate the strange planet so as to try and find the source of the power drain, Barbara Wright stays behind to tend to a disoriented Vicki who seems to be greatly affected by the alien world they have landed on.

As they look around The Doctor and Ian discover that Vortis has a very thin atmosphere with natural crag-like rock formations and pools of lethal acid. The Doctor though recognises the planet from the remains of a dead grub-like creature, however he is puzzled by the presence of moons around what should be an otherwise moonless planet.

Meanwhile, inside the ship, Barbara also becomes influenced by an unknown force through her gold bracelet. This force mesmerises her and draws her outside, leaving Vicki alone in the untended TARDIS. Shortly afterwards large ant like creatures, called Zarbi, then drag the TARDIS away.

The TARDIS Lands on Vortis

Barbara, in a trance, walks into a trio of butterfly-like creatures, called Menoptra, who free her of the trance by removing her bracelet. As they debate what to do with her Barbara escapes but is soon captured by the Zarbi who and brainwash her again through the use of a gold harness they place around her neck. She is then taken to the Crater of Needles and forced to gather vegetation and drop it into rivers of acid, thereby feeding the central force of the Zarbi, called The Animus.

Meanwhile The Doctor and Ian, having discovered the theft of the TARDIS and a trail leading away, begin following the path the thieves left behind. They are soon captured by the Zarbi and are taken to the Carsinome, where they find Vicki and the TARDIS. There they indirectly meet the Animus, who talks to The Doctor through what appears to be a mental communications device.

The Doctor

The Doctor learns that the Animus has invaded Vortis and has ravaged the surface of the planet and taken control of its lower animal life, including the once-docile Zarbi, turning them into its vicious drones, who then use organic Venom Guns to enforce Animus' will.

The Doctor also works out that the Animus uses gold as a conductor to channel its mesmerising force. He counteracts this force and then uses the hidden power of his ring to control one of the Zarbi. The Doctor then escapes with Vicki and his captive Zarbi, and meets up with Barbara, who has managed to escape from the Crater of Needles, and the Menoptra.

The Doctor and his companions realise that not all the creatures on Vortis are being controlled by Animus. The Doctor and his companions find that they have allies who want to be rid of Animus and to see their planet returned to the peaceful state it was in before it was invaded by the parasitic Animus who is slowly enveloping Vortis with its web-like domain.

A Concerned Vicki

Joining forces with a party of Menoptra, and the underground-dwelling Optera - stunted troglodyte descendants of the planet's original Menoptra inhabitants, The Doctor and his companions eventually manage to get past the Zarbi and infiltrate the Animus's web-city. There they confront Animus directly and Barbara destroys it with a weapon, the Isop-tope, which has been devised by the Menoptra scientists.

With the Animus destroyed and the ant-like Zarbi and death-spitting larvae guns become free from the Animus' control peace returns to the planet. The TARDIS is also released from Animus' influence thus allowing the time travellers to leave.